It appears my question didn't indicate well enough why I was asking why some teachers ask students to trade partners and others do not have students trade partners.
I already am a dance teacher, and I am fully behind the idea that trading partners is the best way to learn to dance. I was asking so that I could gather the dance communities feeling on trading partners so that I could better communicate that to my students. A woman called last week to ask if I required trading partners because she just wanted to do something fun with her boyfriend and did not want to dance with other people. Although I will support their desire not to trade partners, I will let them know what they are missing. Thank all of you for your input, and if you have any other comments, please share!
Kevin
PS. I also think that trading lead/follow roles is a very valuable tool, because it makes it obvious what kind of communication the follower needs in order to do what the leader wants. It also helps the follower to understand what it takes for a leader to communicate that information effectivly. I don't have students trade roles in class, though, that would be much too confusing. It does work in private lessons, though.