Dreamsmith

I'd stick with the proper primary for the runes. Plus you can get better armor (elementalists have about the worst armor in the game). Energy storage isn't nifty enough to offset that, nor is it terribly useful if you've got a high Inspiration (that'll get you more mana per battle than Energy Storage would). On many mesmer builds, Energy Storage would be a much bigger waste of points than Fast Casting.

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Sausaletus Rex

Primary attributes come in three flavors :

Absolutely insanely valuable for anyone who's running a character of that profession - Expertise and Energy Storage.

Very nice for the stereotypical member of that profession but too narrow to be of much use to anyone else - Divine Favor and Strength (don't forget, you can use it for the shield if nothing else).

And a nice little bonus but you can put your points elsewhere and not feel the pinch at all - Fast Casting and Soul Reaping.

That's how I'd rank them, if I had to - Exp, ES, DF, Str, FC, then SR a distant sixth.

As for your primary in regards to runes, it depends on the build you're running. Certain professions, certain attributes pretty much require youto max them. The weapon attributes, healing, the elemental attributes, you want all those at 12 if not as high as you can make them. Other attributes don't really need to go sky-high. What's a 14 in Curses going to give you that a 10 doesn't? A few more seconds on your hexes which are going to be removed anyway? Do you really need to crank up Domination so your Power Leak drain 25 energy instead of 18 when it's still an interrupt and you'll still get the casting cost of their skill, too? And that's not even taking into account the splashing of attributes for a moderate to useful skill that doesn't require a lot of points to be viable.

Really, I find it best to consider each profession combination as its own separate class. There are no "pure" Mesmers or Elementalists, there are variations on a theme, different takes on a role. Elementalist/Mesmer and Mesmer/Elementalist are similar but with enough wrinkles to make as much a difference as from E/Me to E/N. There's no one "best" answer, or there shouldn't be if things are in balance (Which they aren't, of course, but they'll always be tweaking things to make them right) so there's as much viability in one as there is in the other. They'll just have different strengths and weaknesses - E/Me is going to have a lot more energy to throw skills around while Me/E is going to cast skills a bit faster while have the runes to put towards their Mesmer attributes and a different suite of armors to pick from.

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In my day, we didn't have virtual reality. If a one-eyed razorback barbarian warrior was chasing you with an ax, you just had to hope you could outrun him.

Ensign

I'm pretty much in agreement with Saus on the order of primary attributes - I think you could debate Strength vs. Fast Casting, though they're different enough that it's a pretty silly debate.

Expertise is insane. If you're using a bow, use Expertise. Get it to 13 or 14. This isn't optional, Expertise is the very basis of the Ranger. It makes otherwise inefficient skills abusively cheap - low energy skills are virtually free. Expertise gives you an effective energy capacity that can make an Elementalist envious, and you get an effective boost to energy regeneration bonus as a huge kicker. Even people who were dropped on their head as babies want lots and lots of Expertise.

Energy Storage is solid but not particularly spectacular - extra energy is great, but the returns aren't so good that you want to be cranking it through the roof. You just need some of it as an Elementalist, to give yourself some room to work with when casting those high energy nukes. Remember that battles are won and lost early on, and having that extra energy up front is sometimes all you need to grab a victory before you even get warmed up.

Divine Favor is really moving up in my book - I'm not about to rank it above Energy Storage, but it's up there. It just has a ton of synergy with the low cost, low recycle skills that the Monk runs on, and the added efficiency it gives you ends up saving you both time and energy. A high Divine Favor attribute tacks more than half of an Orison onto every skill you cast, regardless of line - your Reversals of Fortune heal, your Mends heal, Healing Breeze gets a kick up front. Plus you get access to Signet of Devotion, which is pretty much mandatory on any competitive Monk. You can make a good case for Divine Favor being higher than Healing on a three attribute Monk build - it's just that good.

Fast Casting is an odd attribute. It doesn't do a whole lot to spam skills, but tactically it improves all of your skills, much as Divine Favor and Expertise do. Instead of your skills being sluggish and unwieldly, they just seem to fly out with a Fast Cast attribute. A skill like Energy Tap or Backfire, which normally feels incredibly sluggish, ends up being a nice, usable skill. The trick is that each second in the casting time of a skill is not weighed equally. The longer a cast time, the longer each additional second feels. The difference between a two second casting time skill and a three second cast skill may only be 50% (and even less when you take aftercast into account), but when you're trying to perform time sensitive actions, to slip a skill in while another is recharging or to nail someone before defenses come up, that one second can make all the difference. It's a better attribute than it looks like on paper - not nearly as good as the ones above, granted, but still solid. I wouldn't neglect it.

Strength...well, it's a skill line. If you like the skills in Strength, and there are quite a few to like, then it's an excellent attribute that you should treat like any other skill line. Besides that, it really isn't too impressive. 1% base armor penetration when using skills is really just a consolation prize, not something that you actively invest points for. Strength is really just part of a package deal. You want some of the skills attached to it, it lets you use a good shield, it gives you a little bit of damage, and, hey, you have it if you're going for the heavy armor anyway. It isn't exactly a selling point but it isn't a dump stat, either. Probably needs a boost to make it stand out.

Then there's Soul Reaping. Think of it like the anti-Energy Storage. While Energy Storage gives you a bunch of energy to use as you please, letting you tip the scales from the very beginning, Soul Reaping is both random and sluggish. It provides you with energy, sure, but not when you really want or need it. During the crucial early parts of a match, it does nothing. Once things start dying and the battle starts to turn, it gives you energy - after the enemy you wanted to finish off has dropped, after the teammate you wanted to heal takes a dirt nap. In PvE, where enemies drop left and right, Soul Reaping is a fine attribute that provides you with a wealth of energy to play with. But in PvP? It is the epitome of too little, too late, always showing up at the wrong time to do its thing.

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In my experience, you very, very rarely want to take a class as a primary if you don't want to invest any points in a primary attribute. There's too much raw, passive power there to leave it on the table. You don't always want to crank things up to high levels, but if you don't even want 6-7 attribute levels in your primary you need to think hard about how you're designing your character. If you don't want the attribute, you should do what you're mentioning, and consider flipping the class for the primary-only attribute.

Besides just picking a different primary, though, there are equipment concerns and those aren't something that you can dismiss lightly. You need to take into account what a given character needs to function properly, and how the different classes can help out with that. For example, a pure Mesmer is not particularly attribute intensive. Going from level 10 to level 12 in Domination or Inspiration is a nice bonus, but hardly neccessary - you get a few more seconds out of your hexes, or knock off a little bit more energy with your burns. Not a big deal - so using a Mesmer secondary is perfectly reasonable. On the other side, you have classes like Warriors who really want that 12 in their weapon attribute. On a secondary, that means dumping nearly half of your attribute points into a single attribute just to make it acceptable, while a primary can use their hat and a minor rune to save a bunch of attribute points. Basically you are leaving a bunch of attribute levels behind if you're playing to your secondary, but some classes care more about that than others.

Then there are differences in the armor sets themselves. This really just comes down to Warriors and Rangers, however, since they're trading in pips and raw energy for AL bonuses. If you're an energy intensive character, you're going to want caster armor - if you'd rather be mixing it up, the added AL can be a lifesaver. The only odd choice here is the Ranger, because while their armor is in the middle from an energy standpoint, their primary attribute gives them so much energy that they ultimately end up having the most available. So you kinda have to take everything together.

I wouldn't bother trying to maximize my armor if I was mixing two caster classes, though. All four classes are similar AL and the differences aren't significant enough to base a decision on.

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Ultimately it comes down to the attributes that you want. Weapon masters, Healing Monks, and Elementalist nukers want to be primaries to be pumped by the primary attributes and runes. Tactics, Survival + Beastmastery, Protection, Earth Defenses, and Necromancer / Mesmer attributes are all perfectly reasonable in the 8-10 range, and function fine as secondary class attributes. Some of them even work better with other primaries - such as using Fast Casting from the Mesmer to quickly cast Resurrect or Animate Bone Horror.

Running an Elementalist/Mesmer who leans heavily upon his secondary is a perfectly reasonable thing to do - you don't need the super high attributes, and the extra energy from the Elementalist is always appreciated. However, I do think that a pure Mesmer is better off as a primary. The class does work rather well with Fast Casting, and coupling it with a high Inspiration score lets you make up for the lack of an energy management primary rather painlessly.

Peace,
-CxE

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Don't argue with idiots. They bring you to their level and beat you with experience.