Atahualpa, nice post.
You are of course right that many games have multiple currencies and this is obviously more frequent in those games that are advertised as 'Free to Play' yet having a real-life currency as well (such as TFW), than in subscription based games such as, say, WoW.
The reason behind multiple currencies must surely be that they have different, if overlapping, functions. There might be many reasons for this, and one important one is surely, as you say, to differentiate between paying and non-paying players, As we both seem to think, however, Power and Gold are only superficially different (except for how they can be obtained), making it unclear why both are needed.
As for the Acorn story, I think similar ideas have been suggested before and I think that something like it should work. A Ladder System that can't be abused would be a great help in implementing ideas of that kind.
I wonder if there perhaps are plans for Power that we haven't seen implemented yet. Perhaps jed can enlighten us