For the NES, I'll buy any ole game. I wanted a complete collection, but unless I win the lottery I don't think it'll happen. As for everything else, I buy what I think I might want to play, what is a deal, and what is collectible.
For example, I bought Eternal Champions for Sega CD. Do I want to play it? No. Is it collectible? No. Was it a buck sealed? Yes.
I have a fairly hefty collection and enjoy the the shit out of it. I'll never play half the games I own, but it's fun for me to collect them. I think I enjoy the hunt and thrill of finding a treasure as much as playing the games.