What school do you go to? Seriously, I'd like to know.
Anyway, that is preposterous and simple to debunk. When you press Play in a DAW program all of the tracks are internally mixed with your plug-ins and send buses etc, then sent to your sound card for playback. When you render a mix to disk it's the exact same data, but written to a Wave file instead. If a rendered mix does not sound exactly the same as when played live, then something is wrong with the DAW software, or something is configured wrong.
Please send a link to this post to whomever at your school told you that, and I'll be glad to explain further if needed.
--Ethan
My professor stated that although digidesign and some others say this doesn't happen. Most professionals find it better to mix down to a two track device instead of bouncing because of some loss of quality. I think I didn't clarify what I was saying and the word "ruined" is a little to intense compared to the real extent of the damaged done.