I should only see a user's part message, when they knowingly meant to include a part message... using their own time, energy and effort to hand type that part message. If they cant be bothered to do this, why should I be bothered to read it.
Even a default part message of "good bye" is devoid of all meaning, because it was automated. It would be the same as printing "good bye" on the backs of all your t-shirts, then passively turning and walking away from me in mid conversation. To me, that's more rude than "Talk to the hand."
The only reason someone would want a part message, is so they stand out from everybody else's plain and messageless parts. Lets be unique and different and attract attention to ourselves! Yeee!
/part good bye. ... say it with meaning.
