Recently I've played (read: idled) mostly b-spec.
I got this idea I want to fill my favourites list with interesting rides. That includes Ford MkIV, Fefe P330 and several other +1M cars. For that, I need lots of money, so I rather train me some bobs to grind while I sleep / make food / read /etc. Sure, it wears out the ps3 and it doesn't make much sense to have the game play itself. But grinding Like the Wind myself would be a bigger pain, so I'll do it this way.
I'd say the better question is do you enjoy b-spec (or is there a thread like that already?). The answer is no. It's like an extra job that is quite effortless to do, but doesn't pay that much.