Isn't it the same with humans on some servers?
Come to think of it you are right...
What I was thinking of when writing that part is the situation of going side by side in a turn with the a.i. and mainly I was thinking about the predictability of the a.i.'s racing line (if this makes sense).
Usually this results in some paint exchange, mainly because he's following the racing line and is very predictable, while a human might see me, get nervous and leave very much space, or not see me at all and crash into me harder than the a.i. does.
Except for people I've raced before and know, I'm more relaxed going side by side in a turn with a car in singleplayer than in multiplayer, so I consider this usefull when learning to drive the whole track.
I was not explicit; what I had in mind was rather pointing out the fact that in my opinion the a.i. is good at the very beginning for following around and getting the hang of a track, getting some idea of the speeds needed to negotiate the corners and seeing the racing line in a somewhat real situation.
This is because the "painted" racing line is not dynamic with regards to the player speed, while a car in front seems to be more helpfull.