BTC doesn't lose it's value if you spend it for something, even if at some later point in the chain it converts back to fiat, because if there's something out there that you can spend BTC on, that BCT will be re-purchased to be used for that purpose. It's a funny little concept called money.
Actually, this is all a "funny little concept" called trading. It's irrelevant whether you spend the bitcoins on fiat currency or goods. You will be flooding the market with BTC, and that will push the value down. It won't be easier to find someone who will trade for the full value in goods than in fiat, so instead of getting less fiat you will get less goods. It would have exactly the same effect as printing fiat and spending them on goods.
Or so it would be, if the system was static and we didn't know about the velocity of money concept.