Never used B-Spec, don't intend to either.
Personally I bought the game to drive the cars and race on-line, not to shorten the lifespan of my PS3 by leaving it switched on for hours on end while it plays with itself. I'm not fussed about earning the credits or cars, I've got enough of those from playing through the A-Spec part of the game. Sure, there are plenty of cars I'd like to add to my garage, but I'm happy to do add them whenever the credits are there from normal racing, I don't feel the need to either grind my way to lots of cash or use B-Spec.