Well, it's also beaten out by the whole Strength x 1.5 rule and the x2 rule for Power Attack too. When a THF hits with his main swing, he's going to get his full Strength bonus and Power Attack bonus. The TWF is goign to have to hit with his off hand just to get that 0.5 that he "should have gotten" when his primary hit. And then, of course, TWF's get boned by the Power Attack rules.
And of course you have to keep taking feats just for TWF to "keep up."