this is not the case DPS has tons of variation, there are 7 DPS not the same as 3 tanks, with dps you will get more benefit if you bring other complementary dps that's true but with only 3 choices of tanks 1 being less than the other 2 just makes a clear vision of imbalance classes making 1 class not being optimal when you can pick the other 2, dps don't have this issue, in fact all dps are pretty well balance and regarthless of how you make your composition along as you have your proper melee,range,caster comp you can't go wrong... again this is not the situation with tanks with only 2 spot and 3 tank WAR being an already secure spot giving then only 1 spot for 2 classes now add that 1 of those 2 classes perform better than the other then get 1 class that will get push out over time if this imbalance is not fixed, and It shouldn't be like that, one class cannot out cast 2 other classes, they all should be able to be paired with any composition and anyone with a sane mind will never replace a war with a pld or a drk...
this also applies to astro.
EDIT: @AIRSWIMMER made and excellent point to that just add to this mess...
