For any working-class militant, alarm bells should ring as soon as the archbishop of Canterbury joins the campaign. The proposals are entirely unworkable and fundamentally misunderstand the nature of finance capitalism.
Apart from the arguments above, there is a big question about whether the demand corresponds to reality. The tax was not originally proposed to be a way of punishing the bankers (as the archbishop and the unions seem to think) but rather to throw a few grains of sand into the high velocity world of global finance in order to bring some stability to the currency markets. John Grahl at the AWL's Ideas for Freedom this year, argued that the tax would not just "throw sand in the wheels" of financial markets but would "throw a spanner in the works". Unfortunately his article is not available online: Grahl, J. & Lysandrou, P. (2003) Sand in the wheels or spanner in the works? The Tobin Tax and global ﬁnance, Cambridge Journal of Economics, 27, pp. 597– 621 But his basic argument is that alot of "currency speculation and hedging" which the Tobin taxers believe causes instability, is actually essential to the operation of global capitalism.
As socialists we should not be advocating that we trick the bourgeoisie into destroying their own system. Neither do we want to create a world of self-sufficient national economies. Instead of messing around at the edges of the system, we should build a movement capable of taking control of the wealth that we created.
p.s. when this tax was first proposed in the 1970s it called for a 0.1% tax. they obviously dropped it to 0.05% in order to appeal in the moderate wing!