I play random on team games, never on 1v1, I think playing as Z or T would impede my rise through the ranks of StarCraft.

But honestly, if you can play all 3 races decently, why not? It's a good strat, and makes the other person unable to do race specific stuff (at least until they scout). I've smurfed on my friend's Bronze account before, I went random, guy didn't scout, he was flipping out when I parked hellions in his mineral line like he was expecting zerglings or sentries at his front door or something.

And correct me if I'm wrong, but wasn't there someone in the TLPD who was listed as Zerg/Protoss for Brood War?