I've seen loads of topics of people trying to sell things and saying I want x amount of dollars worth of BTC. Surely if someone is so keen on the idea of Bitcoin fiat shouldn't even be mentioned.
They should be prepared in certain instances to take a fiat loss for their beliefs.
Not really a good argument, at least for now. Bitcoin is volatile. I want $50 of value for my item. It's easier to list in USD for now until btc becomes the standard.