If the death of the physical book ever does happen, I suppose the question of book covers may become redundant. For now, however, they are still really important, and don't let anyone persuade you otherwise!
In academic libraries they mean little: the one where I work actually throws the paper covers away (this is a bit upsetting when they are particularly attractive!) A recent technological innovation borrowed from the world of bookselling, the facility to display images of the covers next to the catalogue records, has caused some confusion - the image is meant to help the user locate the book easily, but that doesn't work if the pretty cover has been chucked out. On the whole, though, users in academic libraries are more interested in the contents than the superficial appearance of books.
This is not so in bookshops, where a good cover judiciously displayed can make a difference to sales. In my public library past we were well aware that the same thing applies to the lending of books. Face-on displays of books made a big difference to borrowing figures. You could more or less manipulate what got borrowed by your choice of items for display, and that choice was I fear very often determined by the quality of the covers. However well-written and interesting a book is, it will not be borrowed by casual library users if it has a plain or boring cover. Sad, but true.
Living in a part of Wales where Welsh-speakers are not well represented among library staff, I tended to keep an eye out for the Welsh books arriving in my branch when I worked in the public library; I don't work there now, but I do use a different branch as a regular borrower, and one thing that caused problems then still seems to now. I would often find myself trying to persuade my colleagues that a particular book, despite its bright jolly cover, was actually a novel for adults; and now as the parent of a Welsh-speaking child in primary school I equally often find racy adult fiction categorised as children's stories, again probably because of the covers. I can only really comment on the Welsh ones, but of course the library has a collection of books in other languages which the staff also don't speak, so the same may be true of those too.
I wonder whether the artwork on Welsh book covers is different enough to be confusing? There are quite a few adult novels concerned with childhood (but from an adult perspective), and these are the ones which are most likely to have illustrations reminiscent of children's books on their covers. Thus, books retrieved from the children's section include Lleucu Roberts' Iesu Tirion and Angharad Tomos' Si hei lwli (both of which have the titles of children's songs - a hymn and a lullaby respectively - as their titles). These books are not meant for young children!
How do we judge what the content of a book may be, especially when we have no plans to read it ourselves? and, moreover, do not speak the language? The artwork on the cover is not always the best way - but perhaps we are so used to judging by image that we have forgotten this.