Well, lets do the math.
[chance of hit]*[average damage per hit] + [chance of miss]*[average damage per miss] = average damage per attack.
Have to include the average damage per miss because that exists these days. For a weapon, lets assume a Fullblade.
We'll use a Fighter, level 6, 18 strength, a +2 weapon, and Reaping Strike. These are choices designed to try to make Power Attack more worthwhile- Reaping Strike's damage on a miss helps mitigate the lower attack roll from Power Attack.
So your Reaping Strike attack roll with this character is +3 level, +4 str, +3 proficiency, +2 enhancement, +1 class = +13. Damage is 1d12+6, 4 on a miss. Versus an enemy with an AC of 21 (reasonable for that level), you have a 65% chance of hitting, and a 35% chance of missing. Your average damage on a hit is 1d12+6 (avg 12.5), and 4 on a miss.
I'm going to ignore critical hit chances, as they affect both possibilities equally.
.65*12.5 + .35*4 = 9.525
With power attack, that changes to
.55*15.5 + .45*4 = 10.325
Adding Weapon Focus, by contrast, gives you
.65*13.5 + .35*4 = 10.175
So here's what we learn.
1. Power attack is useful. It can be used to increase damage, and increases your damage more than other damage increasing feats in at least some situations.
2. But it doesn't necessarily *feel* like power attack. Power attack should maybe feel like, you know, POWER ATTACK!! YAARGH! But its not.
So any new versions should probably take this into account. If you just want to increase damage, the feat already does that.