This book is a very valuable resource for public librarians acting as a reader's adviser ("I like this author, who/what else would I like?"), but I can't imagine anyone having the time to be poring over the pages as a collection development tool.

Genreflecting is also a great reference for writing or literature studies students who are studying genre - along with lists of authors that they can pursue, there are very lucid discussions of the genres, and it's a good starting point for assignments in this area. (I used it for this purpose at Uni, and liked it so much that I tried to buy a copy - unfortunately it was between editions at the time.)