What's it matter, anyway?
Because if you can realize that bitcoin is not currency because it is not a UNIT then you can realize that the US dollar is not currency because it is not a UNIT.
That is what Ron Paul is really asking Ben when he asks can you define a dollar for me. He is really asking what is the UNIT called a dollar equal too?
So, based on the Ron Paul reference, I assume you're making some political argument? Comparing it to the US dollar and saying that isn't currency either doesn't provide me any more information about why it matters whether or not bitcoin is a currency (though I guess it does eliminate a lot of potential reasons - you're obviously not using the term "currency" in any standard sense).
Anyway, now that I see where you're coming from given that you say the US dollar is not a UNIT...
Unit's are definable in objective terms and are a constant. Bitcoin is not a UNIT so it is not a UNIT of account.
How are bitcoins not definable in objective terms (the satoshi client) and constant (there are 21 million of them, of which ~12 million have been discovered).
Bitcoins are just as constant as gold. Moreso, in fact, if you count nuclear transmutation. Is that what the Ron Paul reference was getting at? You think gold is a currency and a unit but dollar bills and bitcoins aren't?
You cant define things in terms of themselves. When you say bitcoin is x about of satoshi it is like defining an inch as a millionth of an inch