While it is true that aggressive play is generally superior to passive play, using any play exclusively can lead to predictability, and being too predictable is far worse than being too passive. A player who is constantly aggressive and plays many inferior hands is called a "maniac", and skilled players will take advantage of him by calling him more often, using Isolation plays, and by other means.
If a player is not aggressive with his weaker hands, the opponents can safely fold whenever the player does bet or raise. The appropriate amount of aggression can be computed via game theory, and depends on the game being played and the tendencies of the opponents.