In such a setting, long-term trade imbalances cannot be balanced out by exchange rate adjustments, so there is a very real danger that countries are drained of money by running net imports in the long term. While this then prevents the country from net importing even more - which is only fair - it also reduces the purchasing power of the country's population internally.
The circulation of money is slowed, causing unemployment and the very bizarre situation where people cannot afford to buy desired XYZ, while at the same time there are involuntarily unemployed producers of XYZ. Everybody's unhappy, simply because the social construct of money ties their hands.
By decoupling from the gold standard, countries left what was effectively a currency union, which enabled them to restart internal circulation.
This is also the choice that Greece has right now, which this supposed "smartest man" does not, or for ideological reasons refuses to, see: leave the currency union to restart internal circulation of a new currency.