> why would anyone want to exchange a reasonable stable national currency for bitcoin?

Name me 10 currencies you'd be comfortable holding. It'll probably be very difficult. Fact is, billions use currencies with high inflation rates and with relatively short lifespans. And it's not just places like Zimbabwe that have many problems besides insane inflation the past decade. It's also relatively developed countries like Argentina that suffer from insane inflation. It's also small stable countries with a nice standard of living ($27k GDP PPP) with an educated population like Cyprus where many lost their savings as bank deposits were frozen or seized.

And that's just protection from governmental malfeasance that hundreds of millions have to be aware of. But it's also the fact that billions are unbanked. We at HN know more than anyone that providing a physical service (like mail delivery) is 1 million times more expensive than a digital service (like e-mail). Same for banking, for vaults, for ATMs, for services like lending, payments, remittance or money transfer. A digital version can be much cheaper, as well as more accessible. (for a bank you often need proof of identity, residence and employment. Good luck when you have no birth certificate, when you live in a slum, when you have an off-the-record job like selling fruit on the corner of the street). Digital banking can bring cheap financial services to hundreds of millions who don't have it, yet have (intermittent) access to networks as simple as SMS or radio, which can plug in to gateways to the bitcoin network.

Next up is us, the wealthy who enjoy relatively nice financial services. Bitcoin can save 1-3% in fees on any transaction. Imagine that there's a 1-3% tax on everything, not value added, everything, every transaction, whether B2C or B2B, and you have a way to remove that. That's very significant. Entire industries run on margins of 1-5%. Amazon's 2013 margin was 0.35%, imagine they could shave off even 0.5%.

Now that doesn't mean we all should actually buy and hold bitcoin. As you say, the volatility dwarfs this percentage. But volatility can be exclusive to investors. For example, Bitreserve (founded by the founder of CNET) lets you lock in the price. That means you can buy $100 of bitcoin, and see $100 on your account, as if it was Paypal. And then you can spend that $100 anytime you want, and it'll always be worth that much. On the backend the service then lets investors create a market to lock in that price, where various investors make short or long bets, a derivatives market can take out the volatility for the customer, thereby allowing people to use bitcoin, save costs, yet be shielded from volatility. That means business to business payments, too.

Besides that there's lots of other things. Like the fact it can be much more secure. Creditcard fraud is rampant as your password is essentially on the card and you have to share it everytime you pay, it's crazy. Chargeback fraud is an issue. Identity theft is therefore an issue. And you can envision crazy things like a Google driverless car paying, on the fly, to another driverless car infront of it, to move away, so that it can take that lane and go faster, because the one in the back is willing to pay extra for speed, and the one in front is okay with arriving a little later. Those kinds of thing can almost only be built on a global, permissionless protocol layer. So indeed none of us may use bitcoin, it may just be a machine to machine currency, it's too early to tell, it can have an impact in many different ways.