Usually, the publisher is banking on name recognition of the "reviewer" to draw interest.

If you've got a horror novel, and Stephen King or Dean Koontz or Hot Horror Author says "it's the most terrifying thing I've ever read", people are going to be interested. The ploy is to get you to think "well, Hot Horror Author scared the pants off of me, so this must be REALLY scary!"

Now, if you take the same author, and put them on a romance novel, it either a) is incongruous enough to generate interest, or b) is a ploy to get you check the book out.

It becomes a problem when someone sees this blurb and goes "who the heck is this? Why should I care about their opinion?"