After reading this a few times. I have come to the conclusion its good. Not super required but pretty good for levels 1-7 or so.
I mean we could just attack 3x or
Power attack and gain the extra die.
Does your dmg modifier from Str or Dex stack on this as well?
No, power attack just adds a weapon die (or a few weapon dice, at higher levels). It does not add anything to your static modifier.
It's good to overcome resistance and try to fish a crit in the first attack or not gamble the -10 MAP using it in the second attack.