I'm not the greatest fan but I need to admit that I have used on occasion. A while ago I did have a book that had multiple titles of the same name and the author was a common name. The only way to get the specific book with the info I had was the ISBN number (public request from a newspaper review). It can also help sort out edition queries when ordering.

Though I repeat not a great fan off myself, but I can appreciate why it was devised. I believe the publishing industry use it more than libraries.