I usually delete every ebook from my ereader as soon as I finish reading it (unless I've taken notes I want to review). It's not like it's gone forever... it's trivial to put it back if I need it.
My ereader is my
bookbag. My
library is on several computers/DVDs/external drives.
I know the device will hold scads and scads more book than I keep on it. But I also know it doesn't have a
limitless amount of storage. I
will have to delete some at some point eventually. And I'm just one of those annoying people who goes ahead and does something voluntarily right now (when it's convenient and simple), rather than waiting until I'm forced to do it (and it becomes a PITA project) later. *shrug*