I play these games and not once have I found myself flying around the Nurburgring in a 250 GTO and thought to myself 'this is so much better because a limited amount of people own this car'.
As far as I'm concerned, all tracks and all cars should be accessible to all players.
Would FIFA be a better game if you had to grind to unlock teams?
Every Forza to date (and GT5) have the very same reward mechanic in career mode. There is NOTHING wrong with that basic system. Now if you want to argue against removing the Free Play option (which doesn't affect your career in F4) then I might agree.