The volatility of a currency is one of the most important metrics.
It really doesn't matter if a currency is inflating or deflating. It just shouldn't change that much over time. The less it changes, the better. (or, if it is changing, then it should at least change consistently. IE: People expect USD to inflate at 3% so it should).
Since no one can predict the value of BTC next week, or hell... even tomorrow's price of BTC... it becomes a very hard to use currency.