Many years ago (over 40) I took lessons in Johannesburg, South Africa, at Wells Sissons studio. There the classes always rotated partners and there were several reasons for that. First there were many single people who were wanting to dance, and they would have been forced to take private lessons without this method. Secondly, what do you do when you are at a staff Christmas dance, and the big boss asks your wife for a dance, and you feel obliged to dance with his wife? (Or her husband - as the case may be). If you have never danced with anyone apart from your own partner you are quite likely to end up a nervous wreck. And another reason is that sometimes you will understand an instruction in one way when the instructor meant something completely different. I remember once where the instructor (Ted Russell) tried for several months to get me to do something, and I always insisted that I was doing it. One day his wife (Gerda Van Niekerk) saw what I was doing and told me to do the same, but in different words. I did it and Ted said that was what he had been trying to teach me for months. But the difference in words suddenly made it clear for me. That is what a different partner can do - explain in different words how you should be doing a particular step. So I would recommend that you try to relax, enjoy the dancing, and never fear a different partner - it can only improve your dancing and confidence in leading or following as the case may be.