Then there is also the question of trinkets and tier sets. Will tank trinkets drop for me a DPS player? (prot / ret). What about stamina? Will the prot / ret spec have a shitload of stamina? Kinda unfair to pure dps classes if that is the case. And what about PvP? So basically tank "hybrids" everywhere? Or DPS that can heal a ton? Will pure DPS specs be glass cannons?
If I have to choose a "stance" then it is basically the same as picking a spec. I can guarantee that the "stance" will have some sort of cast time / cooldown. Just like changing specs have now. So at then end of the day we still have the dps/heal/tank specs... Why bother then?