As mentioned before, the voume of financial transactions is a problem as it contributes to the increasing financialisation of the economy. As in it contributes to increasing short-termism as the goal becomes immediate returns on financial instruments. But beyond that contributes to the world we live in where inflation on products exceeds the actual rate of growth in the economy. In this we not only have a gross increase in inequality based on asset ownership, but we also have the situation where the Financial Services industry no longer serves businesses, but instead, Businesses serve the Financial "services" industry. This creates all sorts of perverse incentives in business and financial decision making (share buybacks, short termism, speculation, and on and on...) which overall lead to a more fragile and slower growing economy.
So, yes, something that reduces the volume of financial transactions at the same time as raising monex for the state is going to be a good idea.
First off, unless such a tax were implemented everywhere, it wouldn't reduce the volume, just affect where they happened. Second, when you consider the adverse effects of eliminating financial service jobs in those countries that adopt it, I doubt it would be a net fiscal gain for their governments. Third, speculation leading to stupid economic decisions goes back centuries, so blaming that on trading volumes is at best an unproven theory. (Fun fact, studies of the effect of the ban on onion futures in the Onion Futures Act of 1958, are inconclusive but several suggest that onion prices were generally more stable before onion futures were banned. The ban itself was passed because of the disruption caused when someone tried to corner the market on onion futures, so clearly better regulation was needed, but not necessarily a ban.)
