I think everyone agrees with this--you still need some paper based reference materials. But you can only carry a couple of paper books, and if you also carry an ereader you can have a hundred more books. You may or may not be able to access them.

Also, a dead battery can be addressed if you have several means of charging it. A broken ereader (either software or hardware) probably cannot be fixed. The same is might be true of a paper book that is soaking wet or that catches on fire accidentally.