I think the whole idea of changing partners after every technique is exactly to avoid getting accustomed to their style. A person on the street won't feel anything like the people you train with in the dojo, and 2 seperate attackers on the street will each have a very unique feel to them. I think switching frequently helps to teach us to learn how to adapt to whatever style/feel the attacker may have. It seems designed such that we will master ourselves rather than learning how to exploit the flaws within each individual (as you won't have a chance to learn in the split second an attack comes in real self defense situation).
In both cases you are given the opportunity to grow and master yourself. In the constant switching of partners I believe it becomes emphasized moreso. In addition you learn to adjust and adapt from various attackers of all shapes and sizes on typically similar techniques.