Well, the primary reason for its coolness is that when a lot of people are talking about books on their LJs or blogs, they often link to them on amazon.com or borders or bn or somesuch - eg, you're not about to give me a link to find a book in the CPL, and matt a link to find it in Princeton, etc - you're going to give a universally usable link. This is just something nifty for the end-reader. :)

Plus, the advantage of using ISBN numbers is that you don't have to worry about proper punctuation! Which in the case of some authors, can be rather complex and lead to much misunderstanding.