If Bitcoin weren't used as a currency there would be no demand for them. They are not useful for anything else. Thus they would be of no interest to an investor.
Really, it has to be both. It is useless as a currency if it has no value, it will hold no value if it is not used as a currency.
I would argue that a currencies' true value is the freedom it offers users in a market. You don't have to trade something to eat for something to wear. You can trade for something to eat with a currency that's only value is that someone can use it to buy something to wear.