Yes, for me, old f1 cars would be a definatly buy reason! Damage, weather, night racing, wtf? All i want is to race cars
Just imagine these two beasts in gt5:
http://www.youtube.com/watch?v=0jfNqTwCQsA (Alain Prost, in an old Turbo Renault - it think its the car of the 82 Season)
http://www.youtube.com/watch?v=Xv8zzLVRF9g (Ayrton Sennas Mclaren from the Season 92

Ive never heared such an nice engine sound!)
Now, talking about the sounds, maybe its better for PD not to include old F1 cars. They will just blame these cars, if they dont get the engine sound right.
Just imagine an old V12 Ferrari F1 Car with the sound of the F2007!