I don't mind forfeits as long as they're not just blatant disconnects because you're losing. If there's literally no way for you to win a match, I guess it's fine, but if they rage and d/c over one bad critical hit or something...
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On normal wi-fi battles, forfeiting is only acceptable if their Pokémon are hacked. It's just a stupid thing to do; makes you seem like a sore loser, gets you banned for an hour, et cetera.
You're thinking of ragequitting- that is, powering off the game mid-battle. Forfeiting is when you hit "Run", and the opponent is declared the winner. I personally think of it as a far more honorable means of ending the battle when you know a loss is inevitable.
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