"If it's all right to use you to refer to one person, why is it wrong to use they to refer to one person?"

Because "you" can be either singular or plural.

"Incidentally 'barbarian' comes from the Greek, who used it to refer to anyone who didn't speak Greek. "

You're absolutely correct -- my abject apologies. Sometimes my brain cramps when I try to be erudite. It's the risk one runs when he's basically simple-minded.