That's, at best, one thing that can separate good writers from average writers. Great rappers don't necessarily need to have complex rhyme schemes, and a complex rhyme scheme does not a great rapper make. Neither does a complex rhyme scheme alone make a great writer, as content and the use of various literary devices come into play as well.

IMHO, the mark of a truly great rapper is the ability to become an instrument on the track while still communicating a great deal in a small number of words. Flow and presence are as important as lyrics and rhyme schemes, and a great rapper is one who has at least 3 out of those 4 qualities at a high level, with the 4th not bad enough to detract from the others.