You have the supervenience[0] of "medium of exchange" and "store of value" reversed. If something is valuable then there will be demand for it. In fact value and demand in the domain of economics, according to Aristotle, are equivalent. (Aristotle also wrote over two thousand years ago that money is not "real" but that's another story.) When there is demand for something it can become a medium of exchange. Asserting that something is a medium of exchange does not make it so unless there is underlying demand for that something.

Primary and secondary is the wrong way to think about. "Medium of Exchange" is a high-level property that depends on the lower-level property "In Demand". And that is why the value of, or the demand for, bitcoin is critical to its success as a payments platform.

[0]: http://en.wikipedia.org/wiki/Supervenience