Interesting, this is the first time I'm hearing about promiscuous pairing. How big the team at your company is and what percentage of time is spent pair programming?
I do consulting, so I work with a lot of different teams of various sizes and configurations. The best teams tend to pair close to 100% of the time and generally have 4-10 developers. Fewer than that makes it difficult to swap around, mix perspective, and keep things fresh, and more than that starts to introduce more complications in planning and coordinating work streams and sharing context.