I dont think it's rude to play a legal move. And you don't have to justify it to anyone, least of all your opponent. He always has the option to resign a lost game.
Under-Promotion? Rude? A Tactic? Or an Opportunity?

What I find weird is occasionally playing someone who starts purposefully throwing away every piece he can as soon as he realizes he is lost. Not in a desperate-sacrifice-to-get-counterplay kind of way... but seemingly just trying to delegitimize the entire game.
Thanks to everyone who has so far responded to my post.
I think that was a delightful story, Paramedic. Instead of saying "checkmate" you just say "beam me up" -- cool new strategy!
Jamie, I am thinking that such a desperate throwing away of pieces was probably to either a) get you complacent and then perhaps sloppy, or b) was an effort to achieve a position where he might get a stalemate.
Or perhaps he should simply not be allowed around sharp objects.

I once played a game where my opponent had me beaten but was behind on time so I continued to play on. He under promoted many pieces to bishops and knights. I wasn't insulted, in fact it was kinda fun. (I will only resign if checkmate is imminent or I am clearly outclassed-which happens more than I like ;)
In a just-completed game, I (playing black) achieved a King and Pawn vs. King endgame, with my pawn safely passed and ready to promote. There was no danger of a stalemate should I rush to promote to a Queen, but nevertheless, I promoted instead to a Rook.
My reasons were two-fold. First, I felt like having some practice with my endgame, to quickly and economically achieve checkmate with less than the "best" piece. Also, my opponent was playing on, so I inferred that maybe he or she also might want some practice in eluding checkmate, or simply figured I might blunder into a stalemate at some point.
Second, I was (by under-promoting) trying to send a message that . . . yes . . . I can finish this game without a Queen, so if you want to resign, go ahead.
The reason for this post is to gather some input from other players. Do you find it rude if someone under-promotes to drag a game out for extra moves? Because it was not my intent to be rude. Do you see it as a Tactic, to try to "earn" a checkmate with less-than-the-best material? Or do you see it as what it was, at its core -- a chance for an opponent to "see the light" and just resign, for an opponent to take the opportunity to say to him/or herself: "This game is over, he can obviously checkmate me with no trouble or else he would have gone ahead and got a Queen"?
Your thoughts would be welcome, and in advance -- thank you for your imput.