The way most class mechanics work in this game, there has to be some form of time restraint. If not, I could take a tank and 3 healers into any fight, rotate heals and win. There would be no challenge. We would never run out of our resource, as they constantly build in combat. The easiest way to make content challenging is enrage timers. Your tank has to be geared enough to hold aggro and take hits. Your healer must be geared enough to heal the tank. Why shouldn't your DPS have to be geared enough to do damage?
This. Enrage timers are there to make sure that dps have a purpose at all. In end game, you'll find that as encounters get harder, people bring more healers and less damage dealers, because they'd rather win the fight guaranteed, even if it takes longer, rather than try to form a real strategy.
You take away enrage timers, people will bring nothing but healers and tanks to every instance and never have a challenge the rest of the time they play.