Why penalities to change stances?

[RyuujinZERO]

rofl... everyone reading only the title and thinking you're being dense

To answer your ACTUAL question, I don't know why they did that. I would guess to discourage stance swapping for extra DPS, since they seem to be trying to encourage tanks to go into tank stance and stay there and not do extra DPS - but here's the thing, not all tanks do. DRK can stance swap without penalty, it doesn't make any sense WAR and PLD do get penalised -- When WAR especially has some combos that REQUIRE the DPS stance and it makes even less sense DRK do not have this penalty when DRK is crazy squishy outside their tank stance, but might be tempted to swap to activate blood weapon ¯\_(ツ)_/¯

Squeenix moves in mysterious ways.