When I perform, I often encourage large crowds to gather around me and my close up table. It is not uncommon for 30+ people to all crowd in, craning their necks to see what is going on.
This is GREAT for my ego and for the atomsphere but when it come to three card monte, the shell and pea game etc. it creates a problem.
Despite having one dedicated player, other 'backseat' players will guess wildly as the position of the ace/pea. This creates several problems
1) It slows down to routine as instead of one guy making the choice, a whole bunch of backseat drivers try and convince him to choose where THEY think it is causing the player to change their mind.
2) If everyone guess somewhere, the chances are that SOMEONE is correct. I.e. if three people each point to a shell, one HAS to be right. This weakens the routine because you end up with a small percentage of smug people who think they beat the game.
3) It reduces the clarity of the act and reduces it to a puzzle. Since other people guessed correctly, the player feels like an idiot. (I.e. he COULD have beaten the game but he wasn't as smart as the other guys).