As for the political point....well, it does seem that white workers gain relative to blacks thanks to racism. It may well be, in fact it probably is the fact, that racism has hindered the strength of the working class as a whole. So from that point of view, the "black & white, unite & fight" slogan makes sense. But that doesn't address the relative gains. Sure, in a nonracist world, workers as a whole would be better off, but that's difficult to imagine and a long way off. So in the short term, the gain to the white worker of knowing that s/he makes more than a black counterpart provides some psychological benefit. Part of the disturbing message of Roediger's Wages of Whiteness is that you have to look within the working class for the origins of racism as well as outside it. That doesn't mean that you have to denounce white workers as a bunch of hopeless bigots, but it does mean that it's wrong to say that it's all a capitalist plot, too.
Doug