Ok, thats it.
I want someone to list EVERY REASON.
In good sentences, in order on how they messed up, I'm sick of random reply to something so dumb.
That's not why so many people want the filter removed in party chat and private messages. Swearing can be used for emphasis to great effect, and is rarely - if ever - going to be used in an offensive context. Because, as you imply yourself, why would somebody swear at their friends? For what it's worth, swearing was originally going to be allowed in party chats when the idea was first proposed.
Moreover, Cubecraft has a very child-friendly and protective atmosphere. Minecraft is, after all, a children's game, and that can be frustrating for the mature audience that Cubecraft has acquired. At the end of the day, if somebody wants to swear, they're going to swear; no filter can stop that. This is not a matter of stopping swearing, but a matter of where we'd rather people swear: controlled party chats, or the public chat.
tl;dr - Mature audiences are being neglected. Swearing would be used for emphasis, not insulting friends. People will swear either way, filter or otherwise, so you may as well give them a private place to do it, thereby keeping it out of the public chat.