It's not about leaving once. It's about the teams that cherry-pick their matches because they're not good enough to win against the higher-skill teams. So they leave match after match until they come up against some people they think they can beat. Also, if you do want to just go in and leave just to get the card, getting banned from it wouldn't really affect you in any way, so just do it. I doubt they'd ban you for doing it once, but even if they did, why would you care? It only bans you from the Trials, not from the whole game. Just know that you'd be totally screwing over the other two randoms you got matched with.