The worst part is that kids are growing up with the impression that the songs on Glee are better than the originals.. My 13 year old daughter already thinks that about a bunch of songs she's heard on the show..
Sadly, I enjoyed the show during its initial first few episodes, but quickly grew tired of it...
I'd be very surprised if most of the young fans of Glee have even heard the original versions of songs, even after Glee performed them.
Back in the 80s, when I was just a young thing from a small town, still in the single digits, I heard this singer named Tiffany on the radio. I heard songs such as "I Saw Him Standing There" and thought it was awesome. If I had even heard of the Beatles at that point, it would have been in passing. As far as I was concerned, it was a pop song by Tiffany.
Later, when I discovered the radio dial switched to other stations, I discovered the originals ... I had no idea that almost everything they played on whatever station it was, were remakes from the 50s and 60s.
In my teenage years, my musical collection expanded quite a bit, for which I'm thankful now.
I'm guessing it will probably be the same for kids now. They hear these songs on Glee (or youtube or whatever) and play them endlessly. At some point, they will either seek out or somehow stumble upon the original, and take it from there. It's exposing them to music they may not have known they liked.