Barro against The Coin: "...it’s still necessary to work toward legislative deals on government spending levels and raising the debt limit. If we need additional tools to give us more time to get to that deal (and we might this year) we should want to pick the tools that do the least along the way to damage institutions and increase the government’s borrowing costs. And the coin simply entails too much disruption to the normal relationship among the Federal Reserve, the Treasury, and Congress."

So, Why Not the Coin?
The strategy involves torturing the central bank into doing things no central bank should have to do.
joshbarro.com