I think perception is much more important politically here. If I'm a politician trying to "stick it to the rich," I want a marginal tax rate as high as possible. I don't really care about how much the rich are paying -- only that my constituents are convinced of my progressivity.
Many of my friends on the left mention the "90%" tax rates of the 1950s. However, they don't realize that with the then tax breaks, this was actually much lower (and it was the rich that could utilize those tax breaks). They want high taxes on corporations, but don't realize that they are the ones paying those taxes. Indeed, for those who have been the most honest with me, they believe that the less the public knows the more the government can take, and that, to them, is a good thing.