You can do the pre-50 dungeons with non-standard party combinations, I've gone as far as Darkhold with a dragoon doing the tanking, though we did need a proper healer to pull that off. It's just that you need, y'know, friends . . . and probably taking some time pre-fight to coordinate strategy.

The trinity exists partially to mitigate having to take the time to get to know other people and their play-styles because you all get dropped into the fight together with clear-cut pre-established expectations of what everyone will be doing.