According to Scott Seaboldt of the rare book department at the Strand bookstore in Manhattan, the presence and condition of a dust jacket increases the market value of a book. There is always a drive to own these books in their original condition, as they were published. I've seen cookbooks that might have sold for 10 times what we mark them at. They've been in the kitchen with rolling pins holding them open, and maybe they have notes scribbled in the margin by a previous owner. Those aren't collectibles, they're utilitarian.
Condition is also a deciding factor at auction.
She says that her favorite customers aren't worried about a book's condition. Her personal collection includes a 1936 copy of the ubiquitous Joy of Cooking by Irma Rombauer. Collectors, like their prized collections, come in all varieties. It's a huge field, and most people find an aspect that's appealing, because collecting goes to the stomach of the matter. What one connoisseur considers a classic, another may find hard to digest.