The way I see it; Is why keep a physical copy of a book you aren't going to read again? Seems like wasted space if you hold on to a book you only read once. Sure there's the exception of the book being an autographed copy or a special edition. But what about the trade paperback or the first book of a trilogy?

First deal with the exceptions:
Say you have the hardcover of a series and a new book is coming out. If your like me, you've probably forgot a few plot lines in between reading other books. Get the the eBook version, I think its so much faster to catch up with than the physical copies. I know I know, I sound like a walking contradiction don't I? I just said physical copies not read are wasted space. But we are talking about the exceptions remember, the UK cover edition of the books, the Autographed copy or the Special edition you might not want to wear out.

Then make a mental or physical list of books you know in your heart you'll absolutely read again.
We all have that one book in our collection that is worn to pieces from rereading, and its usually a trade paperback. And if you don't own a trade paperback, buy it; it'd be a darn shame to have the beautiful hardcover edition worn.

And finally, if you read the book once and it didn't pull at your heart strings or didn't resonate with you, donate it. Give it away to a friend who might like it, donate it to a charity, or even sell it if its still in good condition.
Do the rules apply to them too?
Well I don't think so. In my experience we only get the eBook versions to either read for a review or for a fast read. These are the books we don't really care about buying the physical copy for. And since not so much heart is really in owning the book, why reread it?
 Unless your dealing with the exceptions of course.

But if your not so much into protecting your hardcovers like I am, reread them. If you love the stories, the characters, the world building; it doesn't really matter if the book is a paperback or a hardcover. The story and plot are that matters and its why you keeping going back to it.