Fiat is a store of value, but not a good store, as the value evaporates with time. Other money, with a near stable value, is better. Bitcoin is more risky as it can go down further, but since it can also appreciate, it can be described as stable with volatility, or stable with a risk. But I expect the value to go up as far more likely than to go down, so as a store of value it is far better than stable money.
What you're describing is a good investment, not a good currency. Bitcoin is anything but stable. Stable plus a little volatility is what the price of a bitcoin on Bitstamp has been for the last few days: it's oscillated only a few dollars around $621 or so. For normal currencies, a 1% change is big deal. Look at that article about Venezuela. 60% inflation is insane. Compare that to the 7000% that bitcoin has gone up in the last 2 years. You can't tell me that bitcoin is stable.
I didn't.