My own personal theory about "swear words" is that they were cultivated by a ruling class of days gone by to get used to being told off at a very early age and therefore accepting of authority and rules. Which in turn prepares us for school, work, over-zealous security guards at museums etc.
My first memory of being told off is when I started to copy the fun sounding words that grown ups used and I imagine that is probably the same for a lot of people.
If you think about it, having words you can't say is pretty ridiculous - why invent them in the first place?
Its not really that ridiculous to have words that are reserved for aggression or to describe something extremely dramatic. Saying someone is grumpy paints a very different picture than saying someone is a bitch. I think both are valid words - but if people said bitch all the time - it would take on the same meaning as grumpy... Words are arbitrary - the meaning they convey is not.