Here's why I rip every single CD I buy into MJ.
1. I never again have to worry about a CD being scratched.
2. I can create a truly random mixed playlist of songs without having to wait for some expensive and slow 100 disc cd changer to change discs.
3. Mixed CD's to take in the Jeep on roadtrips.
4. All of my music is readily available to be copied to my portable mp3 player that I take with me to the gym.
5. Cool visualizations.
Not to mention that I can easily call up any song from any album by any artist all from the comfort of my chair using my remote. Because it's all on the computer already, I don't have to go dig out a CD.
There are some cons to this, though.
1. Hard drive space - although with really cheap 80-160 GB hard drives available this is becoming less of a problem.
2. Home stereo systems typically sound better than PC speakers - even this isn't a huge problem as many people are hooking their PC's up to their stereo systems.
3. The PC has to be on anytime you want to listen to music - I leave my PC on all the time anyway so it doesn't make a difference to me.
So even the cons aren't really cons.