How is it that by being a rapper it makes the person a bad actor?
Well, I think JaRule did an excellent job in Half Past Dead, and DMX was great in Exit Wounds (although not in anything else - he wasn't a good addition to Romeo Must Die - like you said, in that case, there was no purpose for him to play that part specifically).
Rock stars in action movies? Would you rather see Marilyn Manson play a corrupted cop? I gotta say that I love rock much more than hip/hop, but what does music have anything to do with acting? Music is sort of like a supporting role - the thing that supports and can enhance how acting is perceived and accepted by the viewers, but it has nothing to do with the quality of the work that the 'musicians' put in as actors.