It makes sense when your reading pattern is either:

A) Books sit on the shelf until you want to look something up in them, at which point you take one down, find what you're looking for, read it and/or copy it down, and then put the book back;

B) You decide to read a book, so you take it down and more-or-less read it through. It's especially handy to have a digital version if you're trying to do this on the train.

But if you're doing the kind of research where you cross-reference a large pile of books, or you're studying intensely from a bunch of books at the same time, paging back and forth in each one to cross-reference it with itself -- i.e. if you're doing academic research or taking college classes -- yeah, e-books are pretty lousy. But you won't be in college forever. And I find that once I've moved my physical book collection around enough times I get pretty tired of having a physical book collection.