I really don't get why people play a single race. Random gives you a lot more game to play. Unless you are trying for tournament level play... I just don't see it.
You need to focus on a single race if you want to get good at the game. I can't imagine making it to Master or even Diamond league if you play random. A few people do it, but I have no idea how they manage. I think there was even a Random player in the GSL one time. Sure, he lost in the first round, but that's still damned impressive.