If someone takes a defensive line, that is fine. If someone does an inside out reverse on me and repassess me, wow, great move man! But if someone weaves in front of me, especially after I have caught a draft and have momentum, well once they will get away with, but more than that and they are going to get punted. Just a little nudge to destablize them in a turn with the intent to not wreck him or me. I don't like blocking especially if we are driving the same car. If we are in the same car, and I am consitently faster, then it is my responsibility to try and get past cleanly. If the other driver is erratic and intentionally blocking, well then they have shown that they have no regard for me or clean racing.
Now, having said all that, I have never actually done it. I said it to prove a point. But, if blocking does happen, I guarantee you it will happen. Maybe not by me, but someone will do it. That is why leagues usually have some type of policy on blocking to keep the mayham down to the minimum.
Driving different car types things get more complicated. For example, LX6 vs the FZ50. The LX6 will be faster in the corners and the FZ will be faster on the straights. This leads to the LX6 guy getting passed on all long straights, and the FZ driver getting passed in the corners. Lap after lap. The FZ will slow the LX6 down a lot, but the LX6 does not really slow the FZ down because it is pretty easy to pass with power on straights. It will get pretty frustrating, but as long as the drivers are not blocking... that's racing.
Pro leagues are a different thing. They race for real, real money, real life stuff. Blocking is dangerous and so is getting punted. If the league wants to draw a bigger, more ignorant croud, then they "let" drivers get away with more bumping and rubbing than really needs to be there. I would much prefer to watch a hard fought battle over several laps where both drivers never tough each other, but somehow, a pass is made. Sure, the leading driver will take a defensive line, but blocking is different.