I think it would be interesting to see how we would develop if we kept to the same partner (or samll number of - say max of 4) for practising kumite.
I have certainly found that I can 'train harder' with certain karateka eg. Lloydey, Frankie and Helen . Whenever you build confidence and trust with your partner, you begin to move faster and with more force as you begin to understand how to interact with each other, you learn each others parameters and limitations.
In this way, I think it becomes easier to learn your own possibilities and limitations which in turn, allows you to concertrate effort in particular areas eg attacking, blocking, punching or kicking.