Some authors--and I believe Jim is one of them--are really picky about which books they write blurbs for. Others just do it whenever they're asked.
Of course, even if you know an author is careful, sometimes the publisher will use a blurb for a previous book on a new one.
So mostly, I just ignore them.
Except when it's an author who writes under a couple of pseudonyms and one pseudonym blurbs another--I find that hilarious and will buy the book simply because I appreciate the humor. But then, I'm a bit of a book slut. I've been known to buy books for less reason than that.
I say, put the blurb on there. Then tell me what your book is about!!
Honestly, I've read almost everything Jim has published. But I've totally by-passed books he has endorsed because there's nothing else on there, or worse, just some uniformative claptrap about being a "true gem of the genre" or something.