I have won all the endurance races other than the 24 hour ones, and only used a single bob each time.
I didn't use an F1 or X2010 for any of them. For two of them, I used an M Coupe (the Z4 type).
Bob does get a bit worse, but he bottoms out at average. He can actually be pretty racey anyway, if he's got a car that corners well. And with a single driver, you have no need to pit to replace drivers when he gets tired. Yes, you do need a better car than the competition, but it's really not that bad. The x2010 or F1 is still a good idea if it's not a timed event, because you'll finish much, much earlier. Otherwise, just put a good tune under him.
I believe this is not an option for 24 Le Mans, but I could be mistaken.
And hell, it's only fair. GT5 makes you drive every mile yourself in A Spec, so it might as well for it for you in B spec.