Because nothing is worse than dancing with different partners, if you dance with one partner most of the time you will get to know their moves as they will know yours.
Bruce, I agree if you are going to dance in competition you need to have the same partner to perfect your moves.
To be a good dancer in a social setting you need to dance with as many people as possible. What you call getting to know their moves can also be a situation where you are each falling into each others bad habits. Dancing lead/follow with many partners, keeps you an honest leader/follower.