Also to Aerodactyl Legend, Ditto can counter one member of a weather team, but he can't counter weather itself. Sandstorm is still a prominent power, and if he takes out Doryuuzu, and suddenly Landlos switches in after you've EQed, you're essentially done. But if CaP 1 switches in, removes weather, slows down Doryuuzu, and can begin to work a counter-offense, you have a successful pivot in the battle's power.
Switch Ditto out and you can use him again for another Pokemon. It can be frustrating for the opponent, especially if you've copied a bunch of decent moves which can hit more than one member for neutral or supereffective damage (in that case, you don't have to switch out yet).