Parasiting aliens... eh. I understand the rule about it and agree with it to stop lamers, but if it happens once in a while (especially by accident, which can happen for those of us with poor aim, lag, or you're trying to figure out that annoying redded-out-weapon bug), I don't think there's a big need for OMGWTFBBQHAXBAN.
Actually, I can see a couple viable uses for parasite, especially if there's someone/something on the team that it's really important to protect or wants to be a rallying point. But again, not unless the team agrees and especially the person, hehe.
As far as pistoling teammates, it shouldn't happen, period. Or knifing. Or biting.
I would also like to say to please bear in mind that people have critical lag spikes sometimes. There's been one or two times that I've ran up to chomp on something, had a lag spike without realizing it, and suddenly everything speeds up and gets back to normal and it turns out I chomped on some guy to kill him. To him, it looks like I TKed out of the middle of nowhere; to me, it was an accident.