I'll at least defend 650B on being a pretty distinct size from the standard 26 inch wheel. A 650B wheel is 25mm larger than a 26 inch one, and 38mm smaller than a 700c wheel.

As for it's particular usefulness, why not? People tout the benefits of smaller wheels all the time; spend time on the folding bike forum and hear people talk about how fast to accelerate they are. Larger wheels also have advantages; talk about how well 29ers roll over things accounts for a good half of the volume of mountain bike publications these days. Why not split the different and have a wheel in the middle?

As far as 650b in particular, the only real advantage it has (over 650A, or any of the other similar sizes) is a romantic backstory.