Xiaoma, I agree completely with everything you've written. But allow me to concentrate on these parts.
I don't think it is important if currency A or currency B is the primary one, or if people enjoy speculating on it. What I find important is the fact that in this "double currency system" only one of the 2 can be used as an actual currency and money transfer, and the other one as investment/speculation/protection/whatever. I don't think Bitcoin was ever designed to be just that, but there is no reason why it could not become one. This is topic for another thread.
That is exactly the conclusion I came to. And philosophically it doesn't seem important which currency is considered primary. But you are correct is only makes sense for a stable value coin to be used as actual currency. If I owe my rent on the 1st of each month, I can't really agree to a 1 year lease priced in volatile coins. Imagine if I agreed to a one year lease priced in bitcoins this past January. WTF!
So if the people who want to use coins to exchange for goods/services have to price and pay in stable coins, what are the volatile coins actually for? They can only be for pure speculation. But speculating on what? It can only be for speculating on how many people want to speculate on volatile coins. But if the only use of volatile coins becomes exchanging for stable coins isn't it fair to call it a (Ponzi, Pyramid, Matrix, Queueing, etc) Scheme?