Once a player starts playing specific position on every shot, there is only one right speed for the position they are playing. I think George's advice was not for those players; he was speaking to the players who tend to use four-length speed on many shots.
There are a few exceptions to this, such as straight shots that can be played with an entire range of speeds to achieve a precise position. The main exception for pro players is the game ball, and often they play that shot with their most comfortable speed, which is not pocket speed.
