The degree to which yen depreciation affects global trade is not what it used to be when Japan had a 10+% share. Its export share is now about 40% of what it was in the mid-1990s and falling.
There is a strong academic case that countries with deflation and zero rates should be allowed to pursue a weaker currency openly as a policy tool. The reason is that there is no real conventional monetary tool they have left and if they are in a true liquidity/deflation trap, adding more domestic liquidity will not have much impact on real rates. The only way to get activity going may be to crowd in both exports and inflation via a weaker currency.
