I play games for enjoyment, if I am enjoying a game then I'll always finish it. If that enjoyment starts to wane or it starts to feel like a chore then I'll shelve it and play something else for a while, and I'll go back when I feel like playing it again. But eventually I finish most games, because I'm picky with what I buy, so most of the games I buy are worth finishing.
I actually find a lot of this article a bit disturbing to be honest, from a gaming point of view, as it seems more like a justification to make shorter, less involved games (which will be sold at the same price of course), and therefore jumping to the conclusion that giving people less game is the answer - thereby shortchanging proper or dedicated gamers just because the casual market has the attention span of a housefly.
I mean think about it this way, if you make a short game then everyone is stuck with a short game for their money. You make a longer game, and the dedicated gamers have something to sick their teeth into, while the casual gamers still get as much game as they want or their attention spans can handle. Screwing over the more dedicated gamers so that you can cater purely to casual gamers demands seems like a bad play to me, especially when that casual market is always in flux anyway, and is just as likely to wander off and take their money someplace else at any time.
The point of games for most people is the journey, and how much fun they have on that journey, not necessarily about getting to the end (so some heavily story based games that is surely a part of the overall satisfaction factor), so to put so much focus on completion percentages and gamer averages is kind of missing the forest for the trees in my view.