I've only been playing on BBO for a few weeks. I only play ACBL games or 3-robot games. I like competitive games, and games are more competitive when you are able to see what conventions and carding your opponents are using. If I look at my opponents' convention cards in ACBL games, about 50% of the time it is the SAYC card. Most of these players are not playing SAYC but have no card tied to their partner. Many times if I ask what the opponents are playing, and I get no response, probably because they are busy and not watching the Messages. I don't think these players are trying to mislead their opponents, but either they don't know how to use the 'Use with partner' feature in the editor or that that feature is not working properly. If we truly do want players to post a convention card when playing in a game, I think we should not allow them to sign up for a game unless they have one, let them choose one of the standard cards, or warn them that the system has assigned a SAYC convention card for their use during the game.
