Rob,
I was also wondering the same as you, as I was working on a newsletter campaign best practices. Somehow, the single campaign answer seems to me also a bit of a waste of possibilities .
I know that in your registration form you can put people into a campaign directly, but you have to assign them to a specific step there, and of course that means that if someone registers today, they will be put into a specific step in the campaign and receive the next email but either you constantly change your form processing step to go to the next email or they will indeed receive older newsletters. I wonder if the changing the processing step option would be something for you, of course you need to remember to do this after your newsletter has been sent each and every time, but then you could keep the same campaign?
Zuriñe