I do this because I'm not sure if the ebook will be there in the future.
This is why I wish ebooks were sold without DRM.
(For what it's worth, there are programs available that make it relatively easy to remove DRM from your purchased books. I use them on every book I buy for the very reasons you outlined.)
It's not just DRM. I'll be the first to admit that I'm terrible about maintaining my digital libraries. I'm slack on backups and I have files all over the place. Also, I don't put much stock in maintaining my digital library in "the cloud." I'm fairly confident that Google, Amazon, et. al. will be there in 15-20 years, but it's not a guarantee and no one knows what will actually happen with all that data once one of them does go belly up. Remember no one though Lehman Brothers would disappear in less than a week.
This is why I wish ebooks were sold without DRM.
(For what it's worth, there are programs available that make it relatively easy to remove DRM from your purchased books. I use them on every book I buy for the very reasons you outlined.)