Significance of negative setup and hold time
Setup times and hold times describe the limits relative to the active clock edge of a "window" within which the input data must be valid for the data to be reliably recognized. The diagram below (you can ignore the bottom Q output part) shows the situation for assumed positive hold and setup times, but you can imagine them negative. 



If setup time is negative, then the absolute latest that the data can become valid is actually after the active clock edge, Obviously the hold time must be positive and of greater magnitude or there would be no window for data validity. 

If the hold time is negative, then the absolute earliest the data no longer needs to be valid is before the active clock edge, so it can change just before the clock edge and the previous value will be correctly recognized. Obviously, the setup time must be positive and of greater magnitude or there would be no window. 

As to possible advantages- think about the situation where you are trying to increase clock frequency as much as possible.