Just following the letter of the law is so huge. Even people who think they're being nice by doing something out of the ordinary make the situation so much more dangerous because now you don't know what's going to happen. Even if they weren't great drivers, the consistency makes so much of a difference.
There's some nuance to it. You're first at a light and a semi across from you is trying to take a left, the right thing to do is to wave him through rather than mat it when it turns green. You don't stop a line of traffic to be nice to someone who can take their own damn turn. As with everything else, people who don't Get It(TM) ruin it for everyone.
