Well, one good reason is that a person who generally doesn't "swear" can then choose to use them very effectively when the situation warrants. If you always go to strongest words when mild ones will do, you're out of things to say when you need to make a stronger/more forceful point.
Perhaps instead of worrying about someone else's free choice to use certain words iver others, it would be wise of you to ask: why would you complain about forced speech patterns in the context of censorship but then be angry when someone uses their free speech differently then you would use it?
