Insider trading does harm people, and does reduce liquidity. Many people who understand classical economics get this wrong [0], because financial markets are a very degenerate kind of market from the point of view of classical economics. The fundamental error in all cases is to conceptualize insider trading as buying from someone who would have bought/sold anyway . This is precisely failing to think at the margin. It…
I do implicitly address these issues in my comment, when I compare public information with insider information. Noise traders are the goose that lays the golden eggs[0]. They irrationally trade randomly in a stock, masking the trades of informed traders. They make a trading loss on average, and these losses provide the profits of informed traders, which gives those traders the incentive for price discovery.
Although the simplest noise trader models can't capture this (as all information takes the same form) [1], public information is much less costly to the noise traders. So for example, noise traders lose a lot more if a company's earnings are leaked to a few individuals, than if these earnings are made public, because in the latter case the market maker can distinguish information from the the noise trader's trades.
So (and again, a model is really needed to confirm this) public information is cheaper in terms of its impact on liquidity, than insider information. One thing I'm not certain of is whether many insiders competing to trade on the same information would be as good as public information.
[0] See the seminal work of Kyle (1985), http://www.jstor.org/stable/1913210 and also the review article http://scholar.harvard.edu/shleifer/files/noise_trader_appro...
[1] Giving the market maker access to a meaningful information set would resolve this issue.