I did this once in college...only once. I agree, it's a stupid thing to do and don't recommend it. I guess I did it because wanted to do something adventurous, feel alive, take a risk... and I was feeling very lonely at the time. I was also drunk...not a good mix with sex in general.
To make a long story as short as possible, the man ended up harming me. It wasn't rape, because I obviously invited him for that purpose. But he did hold me down while biting and sucking my neck very very hard. When I asked him to stop and told him it was hurting me, he wouldn't. He kept doing it. At that point I was starting to fear for my life. When it was over, my neck was raw. He asked me repeatedly when he could see me again, I lied and told him he could see me that next weekend, because I wanted him to leave. My neck was covered with black and blue bruises on the left side the next morning. I never reported it, because I was so ashamed. Of course it could have been something much worse, but this guy obviously had serious problems.
So, take heed, girls who do this without thinking about consequences. You never know who you may be dealing with.