I think the boooooing has a purpose, it allows the performer to know that the public may not agree with the criticism and is on their side. Criticism, no matter HOW constructive, can cut to the core especially in very young people and it can help a little to know that their fans support them.
I totally agree that if you cannot deal with it you need to get out but not everyone gets that kind of criticism in front of 30 million people who vote your future.
You're right, not everyone gets that kind of criticism in front of 30 million people, but I am sure there are at least 100,000+ AI hopefuls who wish *they* were in these contestants' (Kady/Danny/Robin/AllTheDavids'/etc) shoes. I can see your point, Robin, that the booing lets an AI performer know some members of the public like them, but that's kind of what the clapping, thunderous applause, whistling and [Contestant's name] audience-signs are for. The moment Simon even starts to say the least not-positive thing to a contestant, the BOOing starts up. And that other poster is right...Randy encourages it! Honestly, BOO'ing's not so awful, I don't really mind a good BOOOOO now and then but every show the audience just won't stop and they are getting worse as the season progresses. Simon sometimes has very coherent observations to make about that night's performances, specific to the individual contestants. (Yea, and sometimes he's so full of it his eyeballs are brown, but I digress...)
If a casting professional (and let's just say, for the sake of argument, that Randy, Paula and Simon are the casting "professionals" in the AI universe) has a correction to give to someone auditioning, that performer BETTER pay attention. It means a couple of things... 1) The Casting person wants to see how the performer reacts to criticism/feedback. 2) The Casting person LIKES the performer & their audition/performance. 3) Can the performer do a correction mid-stream, can they take the feedback and fix the problems that the casting person is seeing?
If the actor/performer/singer can take the criticism, not get defensive, do the correction and fix whatever issue the casting person is seeing... then they've probably just earned themselves a job, either that day or sometime in the future.
I wish the audience would start BOOOOOOOOing Paula! She's barely there, much less coherent. The band seems to be fond of cutting Simon off with the theme music. They should start cutting Paula off. PUH-leeze.
agnes!
Look, Ma, they've found the Eight Dwarves! 
So I've been on both sides of this issue. 
