"The artists you mention are all already famous and can write off the time spent on doing free mixtapes as a promotional expense."

True, they are famous now, but all of them used mixtapes to become famous. Wiz came up as a rapper through mixtapes. Lil Wayne released "I'm Me" as a mixtape, which was supposed to be "The Carter 3" but it was leaked so he released it for free. Not to mention, he pretty much invented the "free mixtape to use a marketing tool" which I think he did by accident. His music sucks now, but The Carter 2 is a great CD and I don't think I would have found out about him at 16 unless it had been for his mixtape work.