This is the essence of Paul Krugman's argument suggesting Bitcoin's sinful nature. And this evil is defined in a quote Krugman takes from Charlie Stross:
BitCoin looks like it was designed as a weapon intended to damage central banking and money issuing banks, with a Libertarian political agenda in mind—to damage states ability to collect tax and monitor their citizens financial transactions.
It is on this basis that Krugman makes the judgement that Bitcoin is evil. Indeed, he recognises that the debate about the morality of private money is very different from the debate about how (or whether) the private money actually does the job that state money does.
Krugman's core criticism of Bitcoin - and one assumes other means of exchange (or value storage) that aren't controlled by government - is that is undermines big government. Moreover, if private money succeeds (and the jury is out on this) then there is no foundation for monetary systems that drive these big state, big corporation systems - what we can call 'national accounting arithmetic' or 'magic money tree' models.
This is only a moral question is you accept that there is a moral basis for taxation.
Since I don't believe there is any moral case for taxation (as opposed to pragmatic, practical cases) then I see no reason at all to be concerned that private money makes it hard for the government to monitor my financial transactions.
What happens under this system is that government has to make the case for raising taxes - to set the price of government at a level where people willingly pay. Just as importantly, such a change more or less destroys the use of income taxes to promote 'equality'. Taxes become what they should always have been - means for government to provide the services that people require of that government rather than a blunt instrument of social control.
We still have a way to go - these private monies are risky and unproven. But if the direction is towards a smaller, less intrusive and consent-based government then, far from being evil, Bitcoin is a source of moral salvation.