Not to start another editor war, but which one does support cleaning up crappy MS Word and such markup? (I really have no idea and am just asking)
Beyond that though, I think that if people feel the need to write their forum posts outside of the forums editor (as your members already to, since they are used to it), that says there's other problems with whatever editor you were using before as well. How about some feedback as to what their problems with the editor are? Then maybe those can be addressed by the xF team and then, people won't feel the need to write in an external editor and attempt to paste their text into the forum editor.