I'm not sure the credits are a good system actually. For one, they're too easy to achieve. OTOH if they were hard to achieve, and you would actually gain some access by having them (tracks/cars/etc) then for sure some people wouldn't be able to achieve them, either because they use KB, or don't play enough etc. Shouldn't they be allowed access to the content they've payed for?
you could say it's like other single player computer games, if you're not good enough then you won't get to the next stage to see the next level which you payed for too, etc. but I don't think this comparision is good because you pay to race online.
maybe a system where you have to complete, say, 20 offline laps with a car before you could race it online (while offline driving is unrestricted), perhaps with some "final test" that will NOT be too hard to pass. But i don't think that a more severe limitation is in place.
just my oppinion though.