#2 - Feb. 2, 2009, 6:43 p.m.
The key is playstyle. It is easiest to explain with the death knight. The blood tree emphasizes melee combat and healing. The frost tree emphasizes frost damage and control. The unholy tree emphasizes diseases and minions.
To do the same thing with hunters, you'd say that BM emphasizes the pet, MM emphasizes the shots, and SV emphasizes the traps. That's a gross generalization, but you get the idea. If you like to manage your pet, you might like BM. If you want to spend less effort running the pet in and out of combat and want traps to be a smaller part of your game, then you might look at MM, understanding that it probably means more emphasis on your ranged attacks.
But you could PvP (or in the case of the death knight, tank) with any tree as long as you maximized your focus on that role with the specific talents you chose. The trees themselves become more like flavor: do you like Frost, Fire or Arcane spells?
Again, this all assumes that the relative dps, pvp utility, tanking or healing are all equitable, which is something we are trying to improve.
