I will explain how and why I use @Agenda as well. Probably there're will be suggestions on improvments. In general terms I'm a sales. I have to get in touch with people to make progress and get deals. I don't have the final system in my mind by this moment so I need your input here. I have potential sales that stay in Projects list. To move a project to the contract phase I need to meet with customers so I use @Agenda context to put the next planned activity with the customer. For example, @Agenda Mike: develop the need in a Cisco router (when done I will add the next step, i.e. Mike: work out criterias of choosing a router). The problem is a customer usually do not calls a sales and it's the sales responsibility to get in touch. That's why I usually add corresponding @Call context to activate @Agenda, i.e. @Call Mike: arrange a meeting to discuss the agenda.
If you have already a sales system in place based on GTD it would be nice if you can share it with all of us.