The argument by the Keynesians that deficits don't matter and that money printing is not harmful hinges on the fact that we have been running deficits until now and printed money yet borrowing costs are low as is inflation. The problem is that at some point we will cross a threshold where that is no longer the case. The way markets work is that once that point is reached it will be too late. There is no turning back for the Greeks as two years ago everything seemed fine there as well.
One could have said a few years ago that the housing bubble was not harmful, as many did, because as of yet it did not cause any harm. The same could have been said for the tech bubble before the burst. If we wait for markets to tell us that the deficits and money printing have gone too far it will be too late.