Why so many talent trees?

[Artega]

Arms has always been the burst DPS and support tree for Warriors, much like Discipline is the support tree for Priests. That Endless Rage, which was made superfluous when Blizzard caved to lolMS warriors whining about not having enough rage, is terribly weak compared to the ridiculous combination of MS+Death Wish (remember that time, long ago, when Warriors had 52 talent points for a couple of days? Remember how obscenely powerful that combination was?) is just unfortunate.

Fury is for sustained DPS, which is mostly designed around PvE. Protection is about tanking, and while it's certainly a viable tree for PvP when done right, there isn't any PvP gear for the spec (since gear with high armor, stamina, and block value would be Welfare Epics™ that would be viable for PvE as well as PvP.) Arms is popular in PvP because of the superb burst damage and lethal debuff combined with the high HP and armor that a warrior naturally has, plus an unending source of damage in Rage.

Melding talent trees is a pretty stupid idea, if you ask me. They're all distinct, even if delving deep into one over another may or may not be common. It'll never happen, anyway.