Sort of depends. An intact dust cover can increase the value of a book. Sometimes provides a bit of extra information about the publication. For some people, a dust cover is just something inconvenient, and they may be tempted to throw it away.
Books are worth more if they have the dust cover. Collectors of books look for a book to have the dust cover intact, that's why booksellers mention it. If you are just buying a book to read, then it doesn't matter.
If the dust cover is missing from a book then it is the seller's responsibility to disclose that information. Some buyers may only want the book if it comes with the dust cover.
And most hardcover books do have dust covers...just go to any bookstore and you realize that pretty quickly.
Who would steal a book and then remove the dust cover before stealing it?...that really doesn't make any sense.