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.
What about the other way around? What if the T-shirt business applied your logic when bitcoin was $10 worth, if they fixed the price of single t-shirt+delivery at
BTC2 ($20 at that time) how many do you think they'd be selling now?
If you're running a business, you have to adjust the price to BTC/USD rates to make sure all the expenses are covered (payable mostly in fiat).
If you're individual (ebay-like) seller, who doesn't sell for a living (just getting rid of unwanted goods) - you can fix the price in BTC, take the loss if price goes down, but if the BTC price goes up, you still have to be prepared to lower that fixed price (to remain competitive).
This is a very good point. Most people selling things for bitcoin are bitcoin poor and inventory rich. If they were to price things in terms of bitcoin then their sales would be negatively affected when the price of bitcoin goes up.
In order to be able to have a stable price for their goods, a merchant will need to price their products in something that has a stable value (dollars).