FWIW, I see certain advantages of 650b for touring/randonneuring bikes. It's a good way to get a fairly fat (30-40mm) tire without having to really jack the geometry around to make it fit. (650x38 ends up being essentially the same outer diameter as 700x23.)

The point has been made that 26" wheels would accomplish the same thing, and it would be a lot easier for tire manufacturers to tool up to make lightweight 26" road tires. On the surface, this would be true, but the reality is that the makers of 26" tires haven't done it, but the makers of 650b tires have, so there you go.