Dead horse. Timers are pretty necessary for a number of reasons.

One of the major points is it prevents / discourages prolonged AFK.. Without a timer you would generally see more "brb in 20 mins guys! need to have a shower!".
It also promotes a small sense of urgency, I mean the reason you do EXP dungeons is to EXP.... If you have people taking forever and a day to actually do the run then it is highly inefficient or even on-par with other leveling sources.

90 mins is plenty of time to do a dungeon. If you are hitting the limit or getting even close to it, then there's generally a problem. Aside from this, if you are making the time then the fact there is a clock there doesn't really affect you.