I only have about 1500 ebooks so far, but I'm pretty careful about adding what I will have a pretty good interest in reading. So for me, the answer would be no, and doubly so if they were paid for, even $.99 books. I keep all my books on my PC though and only load about 100 or so on my reader at a time now. If I only stored them on my reader (which is a bad idea), then I'd probably delete them. Ebooks on hard drives take up minimal space... filling your reader to excess with them I find just slows down your reader.
Otherwise, depending on how long you live, you might run into the free time to actually read those titles and they may interest you again.