Here's another reason why you should back up your ebooks.
The indie ebookstore CyberRead just announced that they are about to close. They sent out an email today, and they also posted this on their support forums:
Over the past few months we have been urging you to back up and store your ebooks. Unfortunately due to all the changes in the ebook industry and a few failed sales, we are forced to close down our business. We have lost a lot of money trying to grow this business and the agency model was the beginning of the end.
We ask that you take this action immediately and backup any books you have purchased.
TBH, I've never heard of them before now, and that could be a symptom of why they went under.