If you go to a third world country, it's even less convenient to pay for stuff. For example, you can't get a hotel room last-minute with a credit card in Vietnam, where I checked, because they need to verify your ownership/control of your credit card with a pair of small debits, probably because of the fraud/chargeback situation.
Another example, you want to donate to Wikileaks from the US. Not so easy.
Sometimes you really want digital cash you can take and spend anywhere for any reason, and US credit cards or debit cards or other mainstream bank accounts can't do that.
(There still is the problem of needing to use much more secure computers/devices to manage your digital cash than consumers are accustomed to having and using today.)