I think the mistake here is to think about currency purely in a technological way (what can I do with it?). Money is ultimately a social phenomenon. People use money to do stuff in a society, and have evolved ways to make these transactions safe and convenient enough for everyday use. If you store your salary in a bank in USD, there are multiple societal mechanisms that ensure the money are not going to disappear. Can they break down? Sure. Are they likely to break down? Not very, unless all the rest of the society already did.
Now what if you stored your salary in bitcoins instead? None of these societal mechanisms work for you anymore. I am not even talking about taxation and hassles you'd have to overcome to manage a legal business in bitcoins, but what about fraud prevention? Insurance? Safety of transactions? Knowing your money aren't going to disappear overnight? This not something Bitcoin user can have. Maybe with time the society would build such mechanisms, but until it happened, Bitcoin remains a failure as common currency (as opposed to a toy, a speculative investment or a means to transfer money in a private way between parties that are for one reason or another are reluctant to use the banking system).