Pad this pad that... dps is dps, the faster adds die the faster everyone is back on the boss.
Having everyone spec aoe on a fight like beastlord to blow up adds is a different issue. That is simply being inefficient, and your raid's total dps will be lower than if it had a proper balance.
Bladestorm is also horrible on mannoroth anyway, unless your raid is wiping before the final phase... your overall dps will be lower with bladestorm than avatar or bb.
Total raid dps is what matters, not individual dps, and maximizing the former is what's important (aka everyone aoe spec on kormrok for hands = bad, etc.)
Literally the exact opposite is true. Bladestorm (fury) is insane value for 1 talent point and the single target cost for what you gain is small compared to most other specs in the game.