For me, it's not so much a matter of not being able to let go of, it's much more a matter of smart storage.
Yes, it's great to be able to rip your discs to MP3s but your quality goes down (which can be barely noticable if done properly with all the right equipment) but you still also chance losing them or having them corrupted in some manner. It's just the convenience of being able to make that disc much more portable.
Keeping the disc within the case not only protects the original disc and art work, it also helps to maintain the value. There are still a lot of us out there who must have the whole package, not just the disc or a file. At least, for the releases that we personally really value.
Regarding those travel cases, I refuse to use them anymore. Use to use them when I was spending loads of time in Ocean City. Made for an easy way to haul loads of CDs to listen to on the road and while in town. Then eventually I noticed that the inks (or whatever they are) were sticking to the plastic. Not only did it ruin the look of the CD, I'm afraid that it may eventually damage the listening portion of the discs themselves.
So, no more removing the discs from their little plastic or cardboard homes for any long periods of time for me!