Surprised no one has answered this for you!
It is definitely worth levelling it up. Not just because you can leave the game running in the background to earn you credits whilst you look at pron online, but there are also some handy cars available for you to win. Like the Wind for example wins you the Toyota 7 Race Car.
Its painful in the early stages. I vowed in this very thread never to touch it again, but once your driver progresses you do notice the improvement in his driving style. He still has the ability to frustrate, but I've mellowed out with him
One tip I'd have with B-Spec is in the early days, when the car you've given him is SO much better than the competitions, you can tell him to reduce his speed more to keep his coolness level down. Even a reduced speed is faster than the rest of the competition. Then when he's levelled up a bit, put him back in the harder events e.g. the Yaris one.