Not to mention, the content is the center of all that is RapGenius. It is educational/critical analysis of lyrics, written by people.

Would we expect annotated versions of books to be free? No. Would we expect critiques of movies... with the movies themselves to be free? No. That argument just doesn't hold any water.

True, but lyrics are only a portion of a musical work, and in general they're a pretty small amount of text (compared to, say, the median news article). I think RG would respond by pointing out that the volume of commentary frequently dwarfs source material.