While I agree the bonus chests should be granted on something other than speed, I don't think forcing a diversity of jobs is any different from class stacking, especially when you get people with limited jobs available/classes.

If I had to choose something I would rather have something like multiple bonus chests, each is spawned by fulfilling certain conditions like clearing the dungeon of all monsters (excluding summoned minions), breaking/incapacitating Mini-boss/Boss/NM's, making a Boss exhaust their MP to a certain percentage, killing optional NM's, absorbing so much damage from a boss, clearing a dungeon without anyone returning/deaths, and maybe speed on top of that. Each bonus chest has the chance to drop the same gear giving players who can achieve multiple goals a better shot at gear, but players who are slower or can't pull off all the goals will at least have a fair shot at rewards.

Considering the extremities of some of the goals however, it will likely be difficult for even advanced groups to achieve maybe more than two-three bonus chests per run giving a better sense of difficulty and reward for those who can.