“Birds of a feather flock together”, “Two minds are better than one”, whatever the saying the truth holds, we as humans love to be around similar people. A group becomes very powerful when each member is dedicated to the overall group success and therefore the success of each member. As you often find in school and in the workplace, we tend to learn more from our classmates and colleagues than from more formal sources such as teachers and training programs.
It has been the direct experience of Spark Tennis that indeed sometimes less is more. Mixed drill groups are great because if they are variety and the opportunity for higher skilled players to assist and teach by example or instruction lesser skilled players. This behavior though is not as common as are skilled players, so the result can vary wildly from drill group to drill group in this open/mixed format. If the goal is player development and creating an environment in which players can learn (key Spark Tennis principle), then perhaps private instruction and small groups is the best method, with preference towards the latter for group benefits and community.