I'm a book cover designer too... an YES it would be a FANTASTIC idea to have a BOOK COVER option, where the setup and modification of the different book-cover standards would be easier . I'm only suggesting this, because we book-cover designer are some of the designers, who actually can afford to leave ADOBE, because, we are not cross-collaborating so much with other people in our workflow. The switch from IND & PSD therefor would not affect other people really.
AND YES: SLUG IS IMPORTANT. If infinity wants, I will be happy to send some of my documents over for inspection of my workflow.
If you look at the image I attach, you will see, that I use a lineal to make it easier for the production dept. to check the parameters of the cover. This is placed on a slug.
Regards Thomas