In my view, 650B-sized wheels are terrific for two kinds of conditions. One is for riding roads like these without using a mountain bike:



The second is for converting road bikes that would have come with fairly narrow 700c tires and a fairly steep front end, such as the Raleigh Competition GS:



The wider, lower-pressure tires transform the relatively "racey" ride of those framesets and offer a much more comfortable ride.

Neal