I'm not a fan of timers in dungeons, however they need to be there for a very simple reason: Crowding.
In games without timers, you get people that will go into instances and just idle out for hours, or go afk. That's a slot that is being occupied by someone that could actually be running the dungeon in the first place. Without the limiting factor of the timer (which they are now quite generous with) we would have the same issue that plagued early release, initial launch and phases 3 and 4 of beta but for instances only where no one leaves or groups of jerks will lock up an area because they can.
I don't like that timers are there either, but for instances it makes more sense. So long as the timers are fair, I'm fine with them. Giving us 90 minutes is more than enough to screw up a lot and still do just fine