Even though it's more akin to running the gauntlet than anything else, I do like riding in the city. Usually I ride the Marinoni road bike, though the '93 XO-1 used to be the main steed. I found those 26"ers nice right outta the gates, but they maxed out really fast. You just can't ever get up to a good, Aldag-like power cruise on 'em. Ah, turn then to the 700cs - smooth and luxurious but sluggish; better for motoring on a nice 20k time trial.
So I can see the appeal of the 650b. Something right inbetween the 700c and 26" would in fact be perfectly-suited to the urban ride where you never get up to a good cruising speed and where there's a lot of stopping and starting. 650b wheels would provide the right acceleration as well as smooth handling whilst cruising. Ideal, no?
But so impractical. With so much incompatibility in the bike world already, why more?