We can't chose the players when we start a casual dungeon, we can only hope that they know what they're doing. Anyway there's only few players, expecially at high lvl, that don't know what to do or what's they're roles.
You can find healers that don't heal you enough, leaving you die, why focus more to dps than heal.
You can find dps that don't focus your same target and make you crazy to hold all the mobs, but sometimes is also a way for them to inflict more damage, based on different classes and playstyles.
That don't make the trinity system a joke, that only make your work harder. But with a good equipment and a lot of practice you'll be able to hold any amount of mobs, with 2 black mages that focus on random mobs and spam aoe and a healer that dps more than heal.
No one can make a bad player an excellent one in a dungeon run, but you can try to talk and find a way to play with the players you find time by time.