I used to keep everything on my Kindle. Then I had to set it back to factory settings, and didn't bother to load everything again. Now, I have about 200 books on it. I used to have over 1,000 books on it.
I just don't bother to take the time to delete books, and besides, the more books I have on the device, the more options I have. I'm not always near WiFi, and I never know what I might want to read. The ability to have an entire library in my pocket was one of the selling points of the e-reader.