As Kemal Derviş of the Brookings Institution has argued, pretty much any aspect of macroeconomic policy could be construed to affect a country’s trade balance and, by extension, its exchange rate. It is therefore far better to keep such sensitive matters out of trade deals and leave them to existing, separate, diplomatic processes.
If every country tries to race towards the lowest rate, then in the end nobody gains. If you believe there should be no government at all, then fine. But if you believe some government is good, then you cannot have a system that erodes the tax base in all major countries.