I firmly believe that redesigning book covers breathes new life into the work (and as Nathan said, it does give designers work which is a huge plus). The design of each book cover lends a visual interpretation to the written words and will convey an instant impression to the viewer/reader, even prior to delving into the content -- so with each new design, a book cover has the potential to grab a new reader, which I think is key.
I would certainly not be opposed to buying the same book with different covers -- if they really spoke to me. I am not a big collector of anything so each cover would need to be powerful enough to remind me why I enjoy the book itself so much.
As an aside, when Amy mentioned in class how often the book cover and content page design differs as a result of two different designers working on them -- or the designer not knowing the meaning of the story, I had a flashback to elementary/middle school when we read Watership Down -- we were each given a copy of the book, and gracing the hard cover was an image of a ship, I kid you not....
