Beast Mastery isn't worth it. Like you said, you need at least two skills just to HAVE a maintainable pet, let alone the extra skills to make your pet good. Add that to the fact you have no strategic control over your pet's agression, and you will soon find it more of a crutch to keep alive than really worth it.
That being said, Beast Mastery is not a gimped build as you can still kill things with it, but any ranger that wants to be at the top of his own game will not be choosing beast mastery.
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