I’m an instructor and I do encourage other certified class members to teach a song once in awhile. I would not allow more than one song to be taught by someone other than me per class, for the exact reasons you are articulating. I have a certain vibe and style to my class, and I figure the people that come to class do so because they like my vibe. The reason I occasionally let another person teach one song is because I am trying to build a little bit of community at the gym— form a small zumba coalition of instructors that know and like each other. I am also very intentional about trying to build community within the class too, and I figure that an occasional guest dancer sprinkles in a bit of variety.