I've heard of a few instances where people have purchased a CD (or album or tape) but were unable to rip it to MP3 due to physical damage of the original media. At least, that's what they claimed. ;)

I personally rip all of my music -- I have found that the majority of the music available is very poor quality in comparison to what I can do on my own. The original idea that I had was to do something similar to what phildog wanted. Start downloading MP3s, leave it, and have music when I come home. I can't say about anyone else's reasoning, but I thought that it might be easier than sitting there watching the computer rip CDs. The whole "start a big long process and come back" rather than the "start a relatively short process, and sit and watch it so you can start the next one" kind of theory. But the quality of the available music (due to low bitrates, clipped songs, or "underwater" ripping) convinced me that sitting and watching my computer rip a CD isn't all that bad...