GTD and Napster
In addition to the good comments above, I think in general it's a good idea to transfer media to new formats as long as the originals are kept and stored. Not everything on your CD collection will be available as a download. A good example is the transition from LPs to CDs. A small fraction of LP titles were released in CD format, so anyone selling their LP collection (or their turntables) intending to replace them with CDs were out of luck in many cases.
Not to mention that if you rip to MP3 format, you're losing a lot of musical information that was contained in the CD. The only way to preserve an exact copy of a CD onto your computer is to save as a .wav file (.aiff on Mac) or save in one of the non-lossy formats.