Personally I like that Amazon.com holds my books so I can redownload them if necessary. What if I had them backed up on my PC, and both my PC and my Kindle got stolen or lost (airports, I'm looking at you!) I would be S.O.L. on getting my books back.
As for sharing books...you're technically not supposed to share MP3's either. More often than not, sharing takes money out of the pockets of the people who created the work. DRM is cumbersome, but as a writer, I can understand why writers and copyright holders want some sort of assurance that their book won't just be bought once, then spread all around the internet.
Also, and this is an aside more than anything else, am I the only person who never shared their books? I would buy it, read it, and put it on my shelf for later. If I thought someone should read it, I would give them a recommendation, but certainly not give them MY copy of the book!