With me, it mainly comes down to how much I love a game. For instance, FFX is my absolute favorite game of all-time from every genre from every system. I own 5 sealed black label copies of that game, and don't plan to sell any. Call me crazy...3 of them I bought at USA retail myself when I was a freshmen in college, and the others I got later. I just have so many fond memories of that game....
Now, I do also own multiple copies of some other pretty rare games...and as someone else said, it was mainly me holding on to 1 and trying to obtain a nicer copy. Well, take Chrono Trigger...I have 2 copies because I recently got a perfect one. Chrono Trigger is my second favorite game of all time after FFX, so I definitely don't mind owning two copies of it.
It's when people start "hoarding" (when I say hoarding, I mean more than 2 copies of ULTRA rare games) 5+ copies of really rare games (I know a d00shbag who has 8 sealed FFIIs, and over 20 Chrono Triggers - I kid you not) that I consider selfish. This person has NEVER EVEN PLAYED A SINGLE VIDEO GAME HIS WHOLE LIFE, and only collects for the investment, which I think is dumb. I work in Finance, and can tell you there are better investments...
Bottom line...I see no problem owning more than 1 copy of a game...but to a certain point, folks!