I think that there should be an "update api" provided by addon hosting sites - acessing the API would require "premium" membership.
Creating the API would be great but charging money for the API or for the tools that one would create with the API would defeat the purpose of the suggestion.
If wowinterface creates an API and charges a program developer say, 1,000$ for it. How many tools will be written? who would put up that kind of money ?
If wowinterface created an API and gave it out for free but it also included restrictions that you had to be a paying member to download, could you still download addons from the site for free or would you be required to pay even if you didnt use the tool?
If you did, it would be trivial to fix an existing updater to get around either problem.
While having the API would be a godsend, paying for it would not.
It's to bad that the WowAceUpdater was put down, it was by far the best addon updater. The current Curse updater is absolutly horrible.