The reason you can't headbut in the UFC is because Politicians and soccer moms were complaining too much, and the UFC made a business decision to sanitize their sport.
Headbutting was legal up until UFC 15 and no, 9 times out of 10 the fight wasn't over with a headbutt. Knee strikes will eventually be outlawed in the UFC also, so I don't see your point.
Plenty of MMA fans would love to see UFC under Vale Tudo type rules, once more.
There's nothing wrong with a headbutt. It's no more a cause for moral outrage than a punch.
When i said 9 outta 10 times i didnt mean in a UFC fight between two proffessional fighters who are wearing mouthguards and train to learn to deal with with all kinds of strikes. I was talking about street fights and brawls. The UFC thing i just chucked in at the end.
Your forehead/skull is a lot f**king thicker and harder then your knuckles, trust me. If someone landed a half decent headbutt with their forehead driving straight into there nose cheek area it wouldnt be hard to break the jaw,nose and cheekbones with one strike. Good f**king luck trying to accomplish that with one punch, unless its an absolutely wild haymaker.