A freestanding interpretation, such as a comic book adaptation or portfolio or movie or modern production of Shakespeare, is its own beast. You don't see one version interspersed with another.
No, any new interpretation is "it's own beast," as well. It's the same thing. Well, very often it is anyway.
The same text, with new artists interpreting the same text.
The Frazetta Conan covers are a gray area. Books need covers that attract potential readers' attention, and that bravura pulp style was one method that caught on. Although I certainly would rather he had continued doing comics.
So cover illustrations depicting a scene from the book are a necessary evil? Yet inside illustrations don't "attract potential readers' attention." Despite the fact that the above illustrations for Yeates John Carter book, attracted my attention as well as yours. Hell, you even claimed to be interested in them if they were offered separately from the text.
This is not to say a book on it's own could not attract attention, but how many comics that you buy would you still buy if they were not illustrated?