Yes, it worked initially, as those oppressed groups did get some degree of equal opportunities. But today, as it evolved in such a political way, the subtext has changed, different groups of people using it not to gain equal rights, but equal privileges, or rather, supremacy against other groups.
This 'problem' sounds like a near-perfect description of the civil rights movement in the 60's.
I'm not saying it's the correct tactic for all social ills (as seen by liberals), but it has its use and it has worked before. I fear the problem is that the only tool they have is a hammer.