Short Answer:
I like old games better then most new ones, I'm buying retro games now knowing prices will go up, so I can own the games I eventually want to play. 5-8 year old games can be found at a bargin, so I make sure to pick up what I can knowing I can sell them on eBay for a small profit if I choose to later. For new games I'm much more selective on what I buy.
Long Answer:
I have a lot of video games, and most of them I've beaten. I keep games I've beaten because I am a collector. However I have over 100 games in my backlog, and the reason for this is as described in my short answer. I want to one day play super old games like Panzer Dragoon Saga and Secret of Evermore, but I also really want to beat Dragon Quest IX and Golden Sun for the DS. DS, Wii, PS3, 360, and PSP games now are becoming harder and harder to find, and 6 year old games at EBGames/Gamestop are dirt cheap.
So if I buy old games now, knowing I'll play them, I can save a bunch of money instead of buying them when they are rare and not found in stores. If I don't ever make time for them, or find out I don't like them, I can always eBay them, usually for a small profit.
However with your OP in mind, your friend strikes me as a person who buys games new, not to play them right away and store them. That I don't do, not really, I almost never pay over $20 for a video game. I plan to buy a 3DS soon, and whne I buy it I have 5 games or so I also want to buy. I plan to beat these games in 2013/2014 and then return to my backlog. This way I don't buy anymore then I'll have time for.
However I will admit often I pass on a game, not knowing much about it, and then pick up a copy years later. I recently did this with the Ace Attonrey series (which I love now), so I don't play all my new games before going to my backlog.
However in general, I have most of my recent games finished, and I make a good effort finishing what I have for the current gen before jumping to the next gen, and then limiting myself. Also when I buy a game for $60, then I usually beat it within a few months. Some games I buy and beat asap.
P.S. Super Short Answer I like to always have several games on my TDL readily accessable to play. I would always want 20-30 games in my backlog so when I finish what I'm playing and move on, I have variety on what I wish to play. Plus the money thing, 5-8 year old games are a bargin at EB Games, but when they aren't found in stores, especially old paper box games, prices soar.