Not a bad idea. I though of that too. So why not merge the effect of stance in just one so you don't have to change?
Just for exemple :
PLD stance make - 20% damage taken, Auto attack get buffed
DRK - 20% Damage taken, +20% damage done
War +25% hp, +20% heal, +5% damage
But when you think about it, it could be quit high in damage, so just reduce a little the damage output of tank to stay far behind Dps, but it'll be still a buff that can't make Dps outmatched. Something like 5% on tank stance (if merging isn't possible)