While, in the short term, speculation can end up distorting stock values, over longer periods of time the process of speculation (as well as the futures markets) serve to stabilize market changes. They are investing in what they think will improve, and stopping that investment when the believe it has peaked.
Example: as oil soared over 140 USD, futures investors and stock investors began to shift funding AWAY from companies that were making profits based on that price. They predicted, accurately, that consumers would shift behavior and moderate their consumption of this product at that price. By shifting their investing, these investors actually cushioned some of the inevitable dropoff by shifting funding to companies which were bound to pick up business from a market looking for less costly energy.
While I think that markets need regulation -- actually not so much in the way of new regulations but more support to actually enforce the ones we have -- I cannot see how taxing market investment does anything but siphon money from consumers. Capital Gains taxes and the like simply let the government "take its cut," while corporate taxes are passed along to consumers as an expense.
The real role of government shouldn't be to manage the market, but to work (and a lot better than it has) to minimize and punish fraud while providing a maximum of transparency. Caveat emptor will always be relevant, but their is nothing wrong with the government making the buyer as informed as possible.
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