I was surprised to learn that this feature is not available. It seems fundamental. Am I alone?
No. This is an often-requested feature. I am sure it will come soon someday.
In the mean time, you can add other people as participants to the deal.
This would be very helpful, especially when there is a business partner or spouse that we need to track communication with. Adding participants to an activity doesn't seem to capture everything we need from the various contacts.