I see no social benefit that accrues from allowing trading (vs. investing) in the stock market. The usual arguments that traders make in its favor are just a plain attempt at legitimizing their stealing from market participants who are less sophisticated and resourceful.

1) Efficient price discovery - Individual stock prices often move 20,30,50%+ in short periods of time, prices can't be efficient with such high volatility. Trading more likely causes inefficient pricing due to speculations, margin calls, trigger orders.

2) Liquidity - Sure they do increase liquidity in the market but as long as sellers can find buyers I don't see any benefit from increased liquidity other than (3)

3) Bid/ask spread - I'll admit that lower spreads are desirable and traders to play a vital role in keeping it so, but I'll question how important it is for a long term investor if has to cover a spread of an extra 0.1% and then weigh that against the social cost of tens of billions of dollars worth of wealth transfers from retail traders to some HFT shops in New York.