I definitely think that the crowd will silence to give players the best environment to compete in. They dont do it on their own because its all about hype.
First-I dont think there is any argument that can counter the fact that coaching is cheating unless there are rules set in place to direct the actions of coaching. Not talking about crowd, but ppl who standout, and are readily offering advice to a player. Outside influences should not effect the match at all. The crowd-doing crowd things are considered cheerleaders. Cheerleaders are always welcome, especially if they are cute girls. All you have to do is ban giving advice to a player actively engaged in a set.
2nd-The current argument at hand is whether or not its practically ban-able at a large scale tournament.
It comes down to a matter of opinion in whether or not ppl WILL actually refrain from giving advice from the depths of the crowd.
All of that is irrelevant. The point is that coaching should be banned, Its pretty clear that it isnt fair, players should not receive advice from any players once the set starts until its conclusion.
How do you enforce: the same way you would enforce any other rule. Police dont catch all the drug dealers, does that mean selling drugs should be legal? NO! You would remove any advice giver from watching that set, on a second infraction make them leave the building.
When ppl know you are serious they will abide by the rules. If somebody is coaching just report it to the TO. If somebody in a crowd does it and they arent caught, Well dammit they got away with it, but the more they do it the greater the chance they are caught, and will suffer the consequences.
I think ppl will abide by rules just based on the fact that loud crowds have been silenced to give players a better environment to concentrate, but I have only seen this when its been asked.
Having it as a rule can only help, even if its not completely enforced, IT CAN BE. Its not like it cost tax payers money or anything, just the TO's would ask somebody to leave.