Well, it wouldn't be fooling anyone because mirroring someone's language is a sign of interest/fondness and doing so makes someone more comfortable to speak their mind. If my Ts never cursed, I could potentially feel hesitant to curse or I'd feel like I need to follow their language model. A lawyer or a police questioning a criminal or a T needs to make their clients feel safe.

But the thing about it is that it is so subjective that they could really screw themselves over if they say the wrong word to the wrong person. Like if someone were to randomly drop an f-bomb in the middle of a sentence, that would come off to me as relaxed and casual where it would come off to someone else as being highly aggressive and abrasive. I'm sure there are TONS of people on this site alone who would storm out of therapy if their therapist were to say "god you're such a ****ing b-word", but to me and others who interpret that phrase similarly would hear it as a term of endearment or a light hearted joke so long as the inflection was correct (soft constants, not at all accented but not quieter than any other word in the sentence). And when you have one client one hour that would be highly offended or maybe even horribly triggered by that phrase and a client the next hour who would find it comforting and both clients are highly sensitive people... let's just say I don't envy Ts.