It's interesting that the FTT's proponents are mostly politicians, unions, charities or others playing to popular prejudices and sentiment.
However whenever cool-headed technocrats (the IMF, the European Commission, the central banks - US, EU, and UK) have explored the consequences of the FTT, all have poured cold water on the idea.
Fortunately for us, it is the hard logic of cool-headed technocrats that is likely to prevail as no government truly wants to shoot themselves in the foot when push comes to shove and decisions have to be made.