I played around with NoticeCast (NC) almost a year ago. Yes, you can setup rules with Powerplay (and I guess Query) as well as with the NC Authoring tool.
You can think of the two methods in two ways:
1) End-Users: use Powerplay via upfront to create Agents that will alert them whenever a certain condition (rule) applies. The limitations are stated in your question... that an administrator must create the powerplay cube in order for the end-user to create Agents. (forgive me if my usage of Agents is incorrect -- I used to use MicroStrategy tools.)
2) NC Authoring tool is more for Administrators. Among it's advantages:
a) one can setup the alert to go to any amount of users that can be setup in the authoring tool. (Resicpients do not have to be already setup with e-mail addressed in Access Manager)
b) Rules are created directly from the database rather than a cube.
c) it also supposedly allows "event-driven" Alerts, however I think this is a misnomer.
These are just some explanations that I thought off the top of my head. I hope it helps.
gasman