Im a big fan of "fuckpig" as a insult
Im in with the theory that its basicly learnt that these words are wrong, and thus with people reinforcing this attitude that they're wrong everyday it becomes common knowledge. But as i feel art says, if these words are used in normal conversation often they loose there power to shock. For example if you are around a lot of people who swear often you dont even notice the swearing , but if you are around people who never swear even the smaller swear words are "shocking" to them. Because much of the population isnt prone to swearing, especially the ~controlling classes the words are only said in anger usually, and thus seen as bad?
I find that when around people who swear often in thier everyday language, they get a lot more creative when atually insulting someone, than someone who doesn't often swear.
Why do we use them? i think its because we learn that they are bad, and so when we want to be "bad" insulting someone or whatever, we use these as they come easily to tongue. Everyone knows what it means and they sound nice and gutteral as someone said before me.