Well I mean, yes games are an art form, of course, but they are still meant to be played for enjoyment. Which means, if you have a shorter game = less enjoyment. Of course, I'm going to assume that the 12 games are not crap. Trust me, I don't have a job yet, so I don't waste time with crappy games. The last time I spent 40 hours on a "mediocre" game was, um, never. Every game I've spent longer than an hour on I enjoyed every minute of. With crappy games (IE Assassin's Creed) I can't get myself to spend too much time on it.
I'm not saying I'm a time whore, but I'm saying that if you have a good short game and a good long game, the long game wins. It's not entirely important (and definately not important enough to count hour to hour) but I don't like the idea of just throwing it out the window as if it doesn't matter (same thing for the graphics arguement).
Basically, I agree that Portal is good DESPITE its short length, but admit it, it's one of a kind. It's not good because its short, it's good because it's Portal. Any other game that gave me just 3 hours (max) of enjoyment is crap. The main reason I don't like short games only really counts when said game has no multiplayer. With most modern FPS's, the single player campaign can go to hell. I don't care how long it is, I'm going to enjoy multiplayer much more, and spend countless hours on that. If its single-player only, I'm probably going to play it once and shelf it forever.