I don't know. You seem to be taking for granted that profanity is a Bad Thing, signifying immaturity and lack of control and a sick society, as opposed to just a colorful and sometimes forceful means of expression, which perfectly sane and civilized people can sometimes use to good effect. It's not that people NEED to swear; it's just something human beings do, for all sorts of reasons. Including, yes, the fact that expressing your emotions is not always a bad thing. We're not Vulcans and we're not supposed to be.
Look at "This Side of Paradise." Kirk and Co. actually find it weird that the brainwashed colonists never lose their cool, and are relieved when, after after the spores are burned out, the settlers go back to bickering and quarreling among themselves, like normal humans do. I'm suddenly imagining an updated version in which Kirk finds it weird that the colonists never swear -- and breathes a sigh of relief when they start cussing him out at the end.
The way I see it, any "utopian" society in which people seem to have "evolved" beyond profanity is probably more like a sterile dystopia created by brain-warping spores or an insane computer-god.
See also "Return of the Archons," "The Apple," etc. Wanna bet that Landru doesn't approve of swearing?