I have two kinds of agendas.
If items occur to me or come up in discussion that need to be included in an agenda of a future meeting, I can start or add to an agenda for that meeting. Then when it's time to prepare for the meeting, I haven't lost any of these fleeting items.

If I have people who I regularly work with, team members, employees, boss, etc., I may have an agenda list for that person. If I have a single @Waiting for, I won't turn that into an agenda. But if I have multiple items for one person, I'll keep that as a separate agenda list. Or if I notice when scanning action items that I have a lot of items for the same person, I'll try to call or see them and work my way through them.

Before GTD, I had a boss that would turn a single question into a half hour meeting as he rambled on. I would collect a whole list of things I wanted answers to or wanted to discuss and then go in. It would still take a half hour, but I'd have a listful of answers instead of just one. If I still worked for him, I'd definitely have a running agenda going. And he was impressed on my performance assessment at how efficient I was.