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.
Dollars are a more reliable unit of account at this time. Also, when you are selling to a public not yet familiar with BTC it makes the concept easier for them. If I was selling a service such as software with no marginal cost of production only available in Bitcoin most likely I would price it in BTC, otherwise, pricing would be in USD with BTC payment available.
And the merchant has supplier cost which is price in FIAT rather btc. Volatility could prove to be downfall for crypto currency.