I would say, if you take a mixed group, there should be more than one damage dealer for each damage.
In the EA I tried a lot but in the end played a purely physical group, cleric, two-handed warrior, rogue and summoner/ranger. To a big degree that's because I like the according classes and never liked elemental magic that much in games. I'm also not a friend of the surfaces. And with the new visual death mechanic I'm even more dediced than ever to stay with honest physical damage. It's barbaric to burn, poison or freeze people when you can just disembowel them.
Tha said, it's quite often not possible or advisable to focus fire one foe with all of your party members. In such situations mixed groups shine if they can go two on one against the lesser armor each.
