You'll miss them someday, I know that I have been glad I've kept my hands on things, especially when i need to referance something... The thing with games is, once they've hit budget for a few years you ain't gunna be able to get your hands on it again, unless you're lucky.
Unless you plan to keep them for posterity, perhaps to hand onto your descendants or similar, I can't really see the point in keeping games past the point where you are unlikely to play them. I suppose eventually, if kept in good condition, they might fetch a higher price, but then you are still selling them.
I'm not really playing a lot of games at the moment, and there aren't any released this time of year, so I have a bunch to play that I am either half-way through or haven't started. I might go up to my box and type them out, but that seems like a lot of effort.
If folks wanna sell stuff that's upto them, I wouldn't sell a game because I hadn't played it in a couple of years.