The truth is: everyone has to play his part. If one Teammember, only one, is unexperienced or unwilling to do his /her Best its already looking bad for you.
Bad DPS: nothing dies. Bad Tank: Picks up no medals or 10 stacks (Only excusable when the rest of the Team doesnt pick medals at all.) Bad Healer: Well you are fucked.

In some Cases a bad Tank or DPS CAN BE carried BUT with a bad Healer you are for 90% at the loosing side.