I think, especially in emerging markets, merchants will eventually hold bitcoin.
When you start to see large suppliers get into the market then you'll start to see a decrease in immediate conversions.
Sure, BTC might win out of Argentinan Pesos, but it's certainly harder for the average merchant in Buenos Aires to accept and transact using BTC vs. paper USD. The barrier to entry to accepting foreign USD for transactions is 0; for BTC the entry is more difficult (currently).
If Bitcoin is legal, easy, and cheap to use in developed economies, it doesn't matter where it starts being the most useful. It will spread everywhere.