Because DDR3 memory is so high latency it isn't even worth it. There's no speed increase.
Linking to a poor xkcd to reference an old 4chan meme? That's dumb.
Actually, depending on the version, and if you're playing for max score, you want to make single lines for every line except for the last one of a level, where you want to get a double because it gives you the most amount of bonus points, so your score ends up being higher even if you theoretically got only tetrises.