I keep a lot of it. I've not long got an old Ivy Bridge Hackintosh going in an old Cooler Master case, like the one in the photo, that I was never quite able to get rid of because I liked it so much. Some of it goes into other projects (like the aforementioned Hack), helping others when something goes pop on their own systems and very, very occasionally it goes to eBay (which I find a bit of a hassle) or to CEX (where you don't get much but it's less hassle than eBay.)
However, I definitely keep too much. I have a Corsair 800D tucked away in attic because it's 'too nice/potentially useful' to get rid of, even though one of the top USB 3 ports no longer works. I also have a couple of micro ATX and Mini ITX cases still, just on the off chance they'll be useful.