I've noticed if you hire drivers, that you don't get any Driver XP or Car XP... only money.
I could have sworn that in F2 you get some XP even with hired drivers winning races? Or am I just remembering wrong.
Haven't messed with 2 for a while now, but I do remember that as well. You also had a choice of which driver you wanted to hire, which could affect how well they did. Now it's just a simple hire/fire setup.