With the rollout of credit card terminals at fast food spots, I now rarely use cash. As far as I can tell, only small vendors now accept only cash. So, raw cash's utility is falling. But that's only for us non-criminals.
As Stephen Dubner on the Freakonomics blog writes, cash seems more useful to criminals. Since cash is non-traceable, criminals prefer it, but beyond that, cash-based businesses (small restaurants, banks, etc.) should also be more criminally-related than other businesses, as they siphon money into their pockets (like the business owner Dubner cites) or as criminals seek them out to launder their cash.
With criminals being really the only people using cash, we can wreck their economy by flooding the market with fake dollars. As they try to launder their illicit earnings, the banks reject the fake dollars, instantaneously costing the criminals some percentage of their revenue.
As the dollar gets more debased, criminals will need to move to another currency. They can't move to the electronic dollar, as the US gov't regulates the banks and credit card issuers. That leaves barter, or using another currency .
So, North Korea counterfeits dollars, what do we care? ;)