Stunting with different people will make the athletes better as individuals and the team more alike. The individual athletes will become better because they’ll learn to make a wider variety of adjustments and corrections based on the tendencies of their new partners. Stunting is so much about being able to make to correct adjustment quickly that being exposed to more scenarios that require adjustments will better prepare athletes for the competition mat. For example if Amber heels, but Betsy toes, the people that stunt with both Amber and Betsy will be used to make adjustments for either position and more likely be able to hold Cathy regardless of where she is on the heel to toe spectrum.
Changing partners also gives athletes a chance to learn from more people. Having a base work with a new partner provides an opportunity for some detail of the stunt to be performed a little different and turn into a teaching/learning moment.
I also believe having people stunt with different people on a regular basis will make the stunts more similar to each other. If one group was consistently fast and another consistently slow, swapping some of the parts will push them toward meeting someplace that works for everyone.