For books I want to read and reread and keep on my shelf, I much prefer hardcover with dust-jacket. (My mom has a large roll of some plastic material she got from a library-supply catalog that she uses to protect the dust-jackets of our books.) And for books I might want to read, if the book is hardcover, I prefer to read the blurb on the inside flap of a dust jacket rather than printed on the back like a paperback---dust jackets feel much more professional, like a brief mention "this program is sponsored by so-and-so" on a public radio classical music program rather than a blaring and jarring ad on a commercial station.
Basically, if I'm favorably enough inclined to want the book in hardcover rather than paperback, I want the dust jacket.
There's also the fact that sometimes I want to read and reread the story or stories in a book, and art on the cover is distracting; it's very nice to be able to set the dust-jacket aside and just curl up with the book.
And third, while dust-jackets tend to get beaten up and damaged, my casewrap college textbooks aren't in very good shape either. (Not that I got them new.) As a reader, if I'm inclined to keep a hardcover book, I'd rather the jacket than the book take any damage I might accidentally inflict.