I felt the same way. Why should I even bother with B-Spec? And I tinkered with Bob here and there, I got incredibly frustrated at his inability to pick his own nose, let alone drive a car at a reasonable speed.
And then suddenly, out of nowhere it just bites you. You're hooked. For me it just started as a way to earn some credits and different prize cars (PS3 not connected to the Internet), and be able to do other things.
It has for me become a frustration and an impulsive passion. When you have masterfully gotten Bob to win - without giving him a far superior car - I almost feel a sense of pride.
It becomes a lot more involved when you get Bob to the expert races, as suddenly Bob's tyres start wearing out, fuel actually gets used. Do I send him out hell for leather at the start and hope that we can somehow hold onto this lead? Or do you play it safe and wait till the end of the race to make that push.
Sure the payouts are pretty lousy, especially given the length of some f the races, but it's almost like having a protege! He'll frustrate you at times, but he'll also make you proud when he pulls off an insane pass on the outside in the wet in a full on slide only to catch it at just the right moment and go sailing past.
I'm currently at level 26 A-Spec and 23 B-Spec. Haven't touched A-Spec until today, since I started B-Specing, after Bob did the DTM championship. I got jealous and wanted to take the CLK Race Car around the 'ring.