Currently it's hard to see BTC as any kind of currency. People buy BTC to hold as a speculative commodity, not to conduct transactions, as the speculative nature of its "value" makes use as a currency too risky for either buyer or seller. Should you buy your BTC the day you make your transaction and complete the purchase before the BTC either increases or decreases in "value" then at least you get your money's worth, but if that BTC price is going up you are just throwing money away. Similarly, if the seller accepts your BTC and then fails to convert it to an actual currency with a stable exchange value before the BTC value drops, he loses money.

Exchange rate instability is baked into BTC by the finite supply feature, there's simply no way for it to adjust to demand fluctuation which makes certain its price fluctuation, and a currency MUST have an exchange value within a very narrow range to function.