You can get to level 25ish without serious grinding right? I think that's where the casual gamer is invited to stop. Past that, it gets hardcore...
I guess I'm trying to say once you do most of the A-spec with out major grinding, there isn't much left to achieve for gamer Joe. He's got a Lambo, he's got some cash: time for a new game.
If you are gamer Jim, though, you need that X2010 Prototype, you need golds on every event, you need all 1000-odd cars, etc. Level 25+ was made for people like Jim; made for perfectionists.
This make good gameplay for both the casual and the hardcore gamer. Joe can stop at 25 without missing out on too much, but Jim gets to work his way all to 40.
I guess it's about what you seek to achieve in the game. If 100%ing it your thing, months of grinding is waiting for you. If not, good for you: you're not missing too much.
Anyone follow?