I've played a necro secondary on a number of different classes, but not on a dervish (yet.) Here are some necro ideas that I've found generally useful:
Broadly put, you want to use your secondary class to augment what your primary class is good at and shore up its weaknesses. One thing the Dervish skill set is weak on is a way to dump conditions, Avatar of Melandru being the only option. A cheap no-attribs necro skill that meets this need is Plague Touch. Plague Sending (Prophs) and Plague Signet are also useful choices.
Many of the necro curses are ranged, and don't require line-of-sight. These can be used to pull targets and attack in situations where a Dervish would usually be at a disadvantage.
Mark of Pain is a nice "crowd control" skill for melee primaries, especially when you're attacking a boss along with his minions.
Some nice necro enchantments that are generally useful are Order of Pain, Demonic Flesh, and the elite Tainted Flesh. (Be careful about causing disease when fighting humans.)
With just a few points invested, skills like Vampiric Gaze, Well of Blood, and Soul Feast, are very useful for helping keep your health up.
If you have access to the Prophecies necro skills, check out Spiteful Spirit, Spinal Shivers, and Virulence.
My dervish is currently trekking through Prophecies to the Southern Shiverpeaks specifically to cap Virulence. Cause any condition on your primary target and hit them with Virulence. Your primary target will be in serious trouble, and all of his buddies will be suffering from (and spreading) -4 health degen. (Again, be careful about causing disease when fighting humans.)
Hope you found some of these ideas useful. Good luck!
