So this was brought up in a discussion I was having and I ran some quick simulated numbers on it and I thought it would be worth sharing.
I didn't use a lot of data points but I built a few quick stat lines for fighters at lv 1 6 and 11 assuming an 18 starting strength, a great sword, and what weapon runes based on treasure by level were appropriate to made comparisons against the AC's of monsters of equal (standard solo) and 4+ (solo boss) to make my evaluations.
The rules of thumb that my rough numbers generated are Power attack has higher average damage If:
You only have 2 actions to attack with and your chance to hit on your first attack ~=<40%. This situation can occur hard solo challenges with high AC, or debilitation effects.
You have 3 actions to attack with and your chance to hit on your first attack ~=<70% making 1 standard attack followed by a power attack is advantageous. This situation can occur in encounters of lower difficulties.
It is certainly not an all the time thing but depending on your weapons die, when/how you improve your weapon, your modifiers, and your targets AC Power attack can have regular combat applications outside of resistances specifically.
