That's a valid point. I may choose to keep most of the mp3 files I have now, as long as the quality is good.
I feel a bit uneasy about the possibility that some songs could have been converted to 320kbps from a lower bitrate. I think you or some other user mentioned checking spectrum or something using Spek/Spectro? I've found the webpage but I'm not sure if I could make sense of the information given to me by the software
You could always post the image here and people will give you the diagnosis.
As for scratched CD's that you mentioned earlier, there is a lot of redundant data on CD's so they really have to be scratched to hell before they start messing up. And even so they'll just skip and whatnot. Nothing to do with audio quality as a whole, it only effects the moments that skip.