Never really understood why people do that, I mean keeping a handfull of really memorable titles yeah, but not every damn one.
Being a collector, and how little games are worth by the time you're completely done with them.
Plus I dont buy every game that gets released, thats what redbox is for. If I buy a game I know im going to play through it multiple times.
For each system I'll buy the big exclusives such as Halo, Gears of War, Forza, Uncharted, Last of Us, etc. Then the big multiplatform titles such as Call of Duty, Battlefield, Splinter Cell, etc. Then a couple games such as NHL and Drive Club. I don't buy games like Saints Row (Yes, I know its popular but not my style) or Spec Ops The Line. I may rent them but even that's rare now.