Considering how I buy games (I buy one or two, beat them, perhaps platinum them, than buy one or two more). I usually don't have to. The only games I have that I haven't beaten and have no plan to pick back up are:
Shadows of the Damned
Vanquish
3D Dot Game Heroes
Dead Rising 2
Rage
Mostly due to glitches, or they felt like they were missing something. Vanquish, on paper, sounds like the best game ever, but for some reason I couldn't get into it at all. Beat the first boss and never picked it up since. Hell, most of those games I really wanted to like, but I don't
PC games I will come back to every now and then. Steam and it's sales are hard to beat. And my NES/SNES library has laid unbeaten for years. (I haven't played them after the N64 came out), and I hooked both back up about 2 years ago. Beat several of them, but most don't interest me anymore.