Whenever I see this argument, I remember the bard in our group:
Level 12 Half-Orc Barbarian, raging, buffed with Heroes Feast, Greater Magic Weapon, Righteous Wrath of the Faithful from the Cleric and has the Reckless Rage feat. +32 to hit normally, power attacking for full lowers his to hit to 20.
Barbarian: I power attack for full. I roll a 5, I hit AC 27 with flanking.
DM: Yeah, the monster has a AC of 24.
Barbarian: Oh, in that case I hit and crit with my second attack, hit with my third attack, but miss with my 4th. I do 2d6+49+1d6(flaming) damage each attack, so I do 225 damage to the creature this round.
Bard: Umm, don't forget my +3. That means you did 12 more.
DM: Yeah, like it matters. The creature had 160 hitpoints. He's dead.
Cleric: Besides, Bard Song doesn't stack with Righteous Wrath of the Faithful or Heroes Feast. You didn't actually add anything.