In my experience, fighters are the ones who do monstrous amounts of damage, and mages as well as clerics are the wimpy ones who are there to simply Haste and Heal the fighters.
Now, I don't remember how it was on the lower levels, but on higher ones it's roughly like this:
Fighter: Str 28(+9), Greater weapon spec(+4), weapon of destruction +5(+3d6). 4 main attacks, and 3 from the left hand. Each attack does 1d8+3d6+18 damage. Supposing around 4 of them hit, that's 4d8(rougly 18) + 12d6(roughly 42) + 72 = 132 points of damage.
At the same time, a mage can, at best, do around 22d6 points of damage, which is some 77 points of damage. Now, this is likely to be spread to multiple opponents, which makes the combined damage of a mage better.
However, you usually want to produce a high amount of damage to a single foe, rather than low amounts of damage to multiple ones.
And therefore fighters are usually more valuable than wizards.
At least this is according to my playstyle, which may be drasticly different from other people.
The only thing Power Attack would add would be increased miss chance. The higher level foes are so well armored that you can barely hit them as it is, with your +36 attack bonus, minus 5 for each attack.