Pre-Kindle, I would only keep the ones I really liked, knew I'd read again, or maybe were out of print. The rest I either got from the library, borrowed from friends, or bought and then gave away. I have limited space, and got tired of seeing books everywhere. I also, my first job out of college, worked in publishing and at one point I had several thousand books in my parents basement. Never again!
Now that I have my Kindle, I've purged a lot more than I used to. It depends if they are available for Kindle, and if they are affordable. While I have all 7 HP books, i splurged and bought the Kindle editions, so at some point, I'm going to give away the HP books. And while I swore I'd never buy a Kindle version of a book I already had, I discovered I really dislike reading actual books now, so again, if they are affordable, I'll get the Kindle version and purge the books. I almost never buy an actual book anymore, but will get the Kindle version.