(See any youth orchestra at a practice. Those kids can all play their instruments and keep time. Just keeping it together and with the conductor is always shocking at first).
The fundamental problem with youth orchestras is that people practice at home and then try to join together. So...they've probably come in practicing slightly different timings. Also: playing instruments is hard; when you're expending a lot of your focus on playing the instrument, you have less focus for timing. Another problem with youth orchestras is that there's like...50 people. That not only increases chances of individual screwup, but also if the person next to you is a little off, that can throw you off as well.
Compare that to autotune. The background mix is already recorded; you're probably just going to reuse that--so...when you're in practices, you can practice along in a sound environment much closer to your performance environment. (Technically if the youth orchestra has a recording of the piece they want to play, they can use this technique too, but they'd want to coordinate since different recordings of classical music have different timings). Next, while there are damn challenging things you can do with voice, most autotune songs aren't really in the "damn challenging" category. I can sing along timing-wise with most songs I hear on the radio...while driving; it doesn't even take much focus. Third, there's generally only one singer for autotune--you can do harmony with the autotune itself (who needs backup)? Fourth, that person is not going to get screwed up by a human error by the violinist beside them--the recorded mix they're singing over is very predictable.
But...I dunno. To me, the art is in the composition, so I'm not too worried how easy or hard the stage performance may be. We remember names like Mozzart and Beethoven, but I can't name a famous violinist from more than 30 years ago. For a more modern example like...does Krafwerk even have live performances? *searches* apparently yes:
http://www.youtube.com/watch?v=fZT2MrVMHAs but I'm not even sure if the band is visible at all, what if anything, they're doing live, etc....