I had recently thought about this very topic, and having spent just a bit more time than Krugman on it, and knowing a bit more about bitcoin, I find it somewhat interesting that we arrived at drastically different opinions about the morality of it. I have a rather more positive (haha, pun intended) take on it given what has happened with the conventional monetary system, one that Krugman and any mainstream economist would probably agree with: the system is (largely) broken, and we must look for solutions. If Bitcoin is the technologist's experimental solution, it is worth embracing because it delivers on change. Krugman's solutions will focus more on government force as a constraining framework, instead of extensions of innovation---but I believe at the core of it all, the "libertarians" he decries are also the same people he would agree with on the utter folly of our current banking/finance dominated economy.

My expanded thoughts can be read here: https://medium.com/technology-meets-policy/fd097809f88f