I believe that this feature is generally known as 'Grab Bag.' In a Tetris implementation where tetraminos are sampled uniformly, the player is guaranteed to lose. Given a sufficiently long run of Z or S blocks it's impossible to make any lines. Grab Bag eliminates these pathological runs. It also provides an upper bound on the maximum time between see identical blocks at 13 and can make for a better play experience.
Hm. I should clarify. Runs of Z or S don't prevent lines, but they do guarantee that you'll always leave an eye. It is the accumulated eyes that prevent line formation and lead to a loss.
