I don't go to many rap shows, but this seems to be pretty common practice. The last one I went to was the Big Ballas Christmas Party last year and Ouija Macc seemed to be lip syncing nearly everything. Then ICP was the same, and often J especially seemed like he did not even know the lyrics and mimed along. I wish they could do their set with just the instrumental playing and they rap their parts instead of just playing the songs with full vocals and going along with it. 

i cant perform over vocals.. it's too distracting..

im always worried someones gonna snag one of my instrumentals tho... one of these days.. i'll find a balance, with some punch-ins so no one can bite my shit..