I'm very aggressive when it comes to holding the line I've chosen, so if a car is coming in and going to hit me, I'm not going to flinch and it'll mean a collision (especially early on, since I rarely ever qualify for any given race). So that is perhaps a little unrealistic, but if the AI wants to get around me, it's got to earn it... heh.
Also, I'm likely to help an opponent go a little out of their way if things are getting down to the wire. Catching the back bumper of an opponent going into a turn or keeping my car planted firmly in front of a drifting opponent can buy some precious seconds and isn't unrealistic. Professional drivers in many circuits do the same, so I don't feel particularly bad about it in-game.