You think that most national currencies, outside of EUR, GBP, USD, CHF etc, are "sustainable"? Some countries experience severe currency devaluation like 10-20-30% or more per year. Gold, silver, bitcoins, even mechanic or electronic equipment can be a "parachute" to inflation and devaluation.

The price also doesn't matter in transactions that have to do with individuals outside the banking system. For example if a kid wants to sell his game account or a game item to an online friend of him, he can use bitcoin to the equivalent price-target. Say the kid asks 1000$ for the game account in bitcoins... whether it's 1 BTC = 1000$, or 4 BTC = 1000$ (BTC = 250$), it doesn't matter.

Then the kid can spend this 1000$ (in Bitcoins) through the online retailers, or make it fiat or gold/silver through retailers that accept BTC if he is fearful that the price might slide. But the transaction will be at the appropriate value.