I think that's a pretty good idea. Might also encourage more passing in the AIs (One car stays out front an entire race, but has to start another race in mid pack) because it forces them to drive in different conditions, relative to other cars.
+1
As someone mentioned above, it could be great to train the AI, aswell as making some changing in the field, instead of having go to pit and change car for every AI you want to add.