if they added arena they would need to add a second function to everything (talents, abilities) that they could balance without changing the pve function, because otherwise they would have to balance by changing everything about a class every damn time they wanted to 'spice up' pvp. WoW did it and it was horrible depending on the class, balancing the whole game around pvp is crap.
I believe Rift solved this problem using something similar to what I proposed in my last paragraph, I never played it though, so I couldn't tell you for sure. To make Arena work you have to separate PvE and PvP for purposes of balance, it makes no sense to try to balance them at the same time when you don't have to. It also adds a different, PvP way to play each class, adding depth to the game for those who want to PvP in an arena setting without making all of the PvE-ers worry "god I wonder what they're going to eff up with my class next patch."