Why so many talent trees?

[Lissa]

That's not really what I said. I think warlocks are fine with 3 trees - debuff/dot, pet, and direct. Mages are probably fine with three for their different schools. Hunters, maybe 2 - pet and ranged... survival still seems to be the odd tree out. I don't know rogues that well, but maybe two trees for stabby and control/debuff?

The point being, each tree does things in a distinct manner or lends a distinct flavor to the way the class is played. Let's look a Priest for a second since it was one of the two you mentioned. Prior to tBC the trees were a mess, but when 2.0 came in, you saw the distinct use of each tree. While Holy and Shadow didn't morph much, Discipline took on the role that Blizzard wanted. If you look at Discipline up through tier 6 it's all about longevity and dealing with disruptions (only three talents don't fit this model, Wand Spec, Improved Mind Blast, and Improved Inner Fire). As you get deeper into Discipline, Blizzard then throws in a couple weird talents, but tiers 7 to 9 still hold with the basic premise of limiting disruption and providing longevity. If you tried to break this down to two trees it would cause a real mess trying to move talents into Holy and Shadow since there are talents that both specs can use effectively from the Discipline tree.

I started with warriors because that's a class I know fairly well, and trees don't do things in different ways there, the trees do totally different things. Raiding arms and raiding fury are nearly identical, except fury does more damage. PvP fury is just a waste.

Just because one tree becomes pretty much a waste in one style of play doesn't mean it should be removed or combined into less trees. How many high ranking Fire Mages do you see in PvP? How many high ranking Destruction Warlocks do you see? Hunters (ok, that's kinda cruel since the Arenas are biased against them more than anyother class)? So why should Warriors and Priests be any different in this regard?