Books should be shelfworthy.
Edit: Do not misunderstand, I am the proud owner of many a paperback book. They provide less risk when reading in the bath, and are easier to snuggle up with in bed, and, more importantly for nearsighted folk, to hold next to the pillow. However, the ease in shipping Will mentions highlights one aspect of paperbacks I find unappealing: their transience. A softcover has less sense of permanence to it; a beloved and battered paperback held together with tape and prayer exists at the margins, in a constant battle to keep itself together. The weight and solidity of a hardcover emphasize that it will always be there for you to enjoy.
Update: my college friend Chris, a self proclaimed "amasser" of books and movies, comments. He is correct that I am exceedingly picky about what books I choose to purchase, but this is less aesthetic snobbery than extreme frugality.