MP3s are convenient only for listening to music in cars or on computers. At home, it's hard copies all the way.
1. Sound quality is vastly superior. MP3s sound flat and you can even notice the difference in the car. You get used to it, but certain music requires the full sound CDs, vinyl, etc. give.
2. The artwork. The entire release is, ideally, a work of art. If you don't have the physical booklet, inserts, etc. you're missing a huge portion of the art.
3. This argument of mp3 vs. CD occurs almost exclusively in rap/pop genres. In other words, if you see the demise of physical music releases, it will likely be in these two genres. Many other genres are so intricate and heavily layered that mp3 cannot do them justice, and like the booklets and inserts, a huge portion of the work of art will be gone.
That's my take. MP3s for convenience, hard copies for the experience. I would NEVER actually pay money for an mp3.
EDIT: One last thing, digi-packs suck. They're cheap and I've had some fall apart within weeks. I've gotten a few quality digi-packs in my day, but by and large I take jewel cases over digi-packs. Plus if your digipack is fucked up it's fucked up. If your jewel case cracks, you can just replace it.