I suppose one reason why hardcover books have dust jackets is that you can treat your expensive hardcover book like a inexpensive softcover. With such a jacket it can be used as a convenient coaster for sweating drinks, to swat flies, and to some extent a portable surface on which to write.
A
valued softcover book on the other hand does not benefit from any of these advantages. A softcover book would merely absorb the sweat ring from a coaster, fly guts would litter the actual cover, and as a writing surface it's completely inadequate and impressionable.
A Hardcover Book Jacket would be a retrofit for a softcover book and consist of an adjustable spine to accomodate various thicknesses. The length and height of the hardcover would be standardized to fit most softcovers with minimal empty margins. The flimsy covers of paperbacks would insert into slots in the Hardcover Book Jacket for easy installation.
As a secondary function, the paperback can be removed from the book jacket, and the book jacket placed on its back cover. With the covers opened, to between forty-five and less than ninety degrees, it can be used as a fine book stand for comfortable reading.