Make it optional, but give us added incentive to try it. For example, as other people above me mentioned, use the Bobs we develop to take on segments of endurance races so the player can take them on at something approaching a sane stretch of time. As for another added incentive, why not allow us to do what the original Forza Motorsport allowed us to do with our Drivatars and go toe-to-toe with our own creations?
Think about it like this: since we trained and raised our Bobs I say we have the right to drive with/against our Bobs (or hell, others' Bobs) to see whose Bob is the Bobbiest and what kind of opponents they really are. It would be great to see these distinct Bob-personalities with their developing strengths, weaknesses, and pseudo-preferences all on the track facing off against each other. Racing against Bobs should also train them somewhat, so leveling up a Bob isn't just grinding while watching races happen, and it's nowhere near as frustrating as the Forza Drivatar method, which had me in fits trying to go around corners perfectly to teach my Drivatar how to round one without crashing in a heap.
And, when your Bob reaches Lv 40 and is dropped from your Bob-raising rotation in B-Spec, we ought to have the option to keep our Bobs for this purpose. This way we get to keep our Bobs and immortalize them in a way.