Yeah, 650b has no actual advantages over 26" or 700c. It has gotten popular recently because the French constructeur bikes were often 650b, and developed a sort of cachet in the retro-grouchy crowd that wanted an Alex Singer but couldn't afford one. Kinda like the 700c wheel did to the 27"- there's nothing wrong with 27", but it got associated with cheap steel-rimmed junk. It's all image and very little function.

That being said, I am building up a Raleigh Portage, one of the few 650b bikes available in the US at the time, and I won't feel self-conscious about riding a bike with drop handlebars and fat tires when I do so. Most bikes built for 700 wheels won't accept larger tires and MTB frames have a different geometry.