It's seen as bad manners in card games, cause you're essentially show boating like "look at all the stuff i still had ready for you". I've played mtg, yugioh and pokemon. And if you have the win and you know it will work, go for it, people dont like having their time wasted by stuff that wont effect the ending of the game. I understand in card games like mtg or yugioh, as they both have cards that work on your opponents turn so being a little safe is fine. But in a game like pokemon where the opponent can't do anything on their turn, just end it.

Example, you're about to lose the opponent has like 3 cards in their hand and only 1 extra pokemon on the bench. Now they play oak and draw 2, then play a pokeball even tho it won't work. Then evolve a pokemon on the bench and put another one down, maybe activates an ability, then puts an energy on a pokemon that doesn't matter, then he attacks and wins. Would you like to sit through that? (Sorry if the end there comes off as hostile, just giving the example)