That has not been the actual experience.
"Reasonable" is an added herring. Fair use is fair use. Book covers should
be allowed to be used, without restriction. All book covers are fair use.
ITYM All book covers are fair use on an article about the book itself.
Which is probably true; I can't really think of a counterexample.
But that does not mean you can use the image without restriction; a fair
use claim always refers to the exact use we make of it.
Can you point us at book covers that were tagged as fair use, used on an
article about the book, and still were deleted?