well, i've had some experience of both sides of this coin.
Certainly, for those just starting to dance and learning the basics, it's an excellent idea to swap partners, otherwise each person in the couple can compensate for mistakes made by the other, and bad habits can build up.
On the other hand, as you've said, many adults wouldn't normally want to swap partners, and for a couple that are progressing well, and have potential, being forced to swap can be a nuisance.
Teachers should always encourage their pupils to dance socially. When i started, i was fortunate that my teacher organised her own 'socials' with other classes. That helped me a lot