In most board games, proxy pieces are generally forbidden in official play. For say, a card game it's because the game store owns the cards and would really prefer it if you didn't dillute the value of the cards they're also selling to people, not to mention they also have the actual cards in stock anyway. On a similar note, chess tournaments and clubs will almost always have enough pieces to not need proxies since there's only 4 unique pieces that you'd potentially need to add for (all the officials minus the king, in practice it's usually only the Queen and the Knight though), so any extra/reserve chess set can provide the bonus piece.

In casual play outside of formal tournaments and chess clubs, proxy pieces exist because nobody is buying extra chess kits solely to cover for the event in which someone promotes a queen while another queen is on the board. (Also in very casual play, most players lose their Queen due to a mistake early on and if they promote a piece to Queen later, they just use the original Queen piece again.) Proxy pieces exist to cover people playing at home, not people playing professionally or at a hobby club. The same goes for card games; nobody cares if you're proxying a card during casual play - maybe they'd ask if you own the card, but that's about it.