The Tobin tax as it was originally conceived is a knee-jerk reaction to the problem of speculation. It will harm all market participants, both speculators and non-speculators, because both will take actions to avoid the tax. The transactions cost incurred for avoiding the tax, coupled with the tax itself, are harmful to the market.
The problem is speculation, and I don't claim to have a full-on solution, but see the * below for a solution idea that may be a bit of a tangent.
Even so, I do aplaud Bill Gates and other leaders for discussing things. There may be some good to come out of all of this discussion, but I don't think the Tobin tax or any similar tax is one of them.
*Permit currency creation: For instance, permit corporations and persons to create their own "currencies", backed by whatever they see fit (oil, gold, silver, other commodities, a basket of Government currencies, their own bartered labor, etc.). I don't have the entire solution here, the idea would need to be refined, but it could bring a significant net gain to the currency problem. I mean, we all know and expect the dollar will "die" some day - in the end it is only a piece of paper or a bank balance - and so this could replace it...