The disparity is certainly there but it's not a fair assumption to blame it all on race. Cocaine use vs crack use breaks down along socioeconomic lines. Because crack is more prevalent among impoverished communities, there is more street crime associated with that drug.
The huge social problem that came with crack is the fact that it was the first drug that became very popular among women and created a large population of female drug addicts. This in turn left thousands of children neglected because their mothers were strung out.
In short, the effects of crack are much more harmful to society than the effects of cocaine. I believe the longer prison sentences for crack are an attempt to reign the problem in; albeit an ineffective effort.