Just calculating a skills's worth based only on damage per AP spent kinda ignores the reality of the actual gameplay.
Let's say your 1H warrior gets 7 AP a turn. If you have 4 AP left on your 1H Warrior's turn, you only get to attack once anyway. You could either do a power attack now, or save the remaining AP up, but on your next turn, you'll still only have either 7 or 8 AP - two attacks only, either way. Doing two normal attacks will leave you with 2 AP spare, enough for 3 attacks the next turn.
I'll grant you that an enemy at high health is not the best target for power stance attacks, but at low health, sometimes the extra AP spent can be useful if you want to get an enemy down faster.
