I suspect you think the hard-left fear it because it might work, but the problem is mroe the perception that it's a red herring. Assume for the sake of argument that exchange controls were generalsied. Then you would be back to Bretton Woods, yes? I would say it's impossible to unmake the omelette we now have, but in any case even if you did, whatever stability you achieved would only be a short-lived prelude to some mroe explosive contradcition down the line. Exchanmge controls are (the libertarian/globalist critique is correct) a beggar-my-neighbour kind of policy. They work by unloading the problem onto some more vulnerable place. And if your assumption is that they work on an optimising welfare basis, eg by achieving all-round growth, I think this is the most serious error of analysis.
Mark