I agree that YouTube can be a great resource when used thoughtfully. I direct my baronial choir, and in recent years I have been sending them links to YouTube recordings of better quality performances of the pieces we're doing.
I'm not convinced that they help people learn their parts, so much, but they can be of aid in understanding how a piece should sound overall and also (if you're careful which performances you choose) help people with pronunciation of foreign languages.
Of course, as with everything on the Internet, you have to be careful which performances you use as your models. A high school choir in Nebraska? Maybe. A college choir? Probably better, especially if it's from a college with a respected music program, like Indiana, Oberlin, Juilliard, or one of the conservatories. A well-known professional group that does early music, like the King's Singers, Tallis Scholars, or Chanticleer? Yeah, I trust those, but still listen to multiple recordings when they're available so I can compare and consider which ones sound best to me.
From: Daniel Patrick Hall <dan21may2011@...>
Sent: Friday, September 2, 2011 8:58 PM
Subject: [KWChoir] Byrd - Ave Verum Corpus
Do we use this resource for lots of stuff such as this great song, already?
Byrd - Ave Verum Corpus