Actually in a pld/war composition it's not too big of a difference between having either of them as MT. In a12s it's probably better to have the war take more tank busters than the pld (holmgang - thrill/venge - swap to pld - holmgang - thrill/venge). Drk is a lot more inclined to the MT type since it has two dps skills based on parry procs and mp regen skill based on taking damage.

Anyway I'm not against what you suggested, having all 3 tanks viable for either slot, I'm just saying that's a lot harder to balance, and a lot more things can (and probably will) go wrong. Since we don't get a new tank in 4.0 I guess that's the road SE is trying to follow, so let's just see how it goes.