Dylan Matthews of the Washington Post's Wonkblog officially joins the cause.
They are justified as providing an even playing field for small investors, but obviously such a playing field doesn't exist. You really don't stand as good of a shot of beating the market as a guy with an E-Trade account as you do when you're, say, Steve Cohen. So you shouldn't try. Making insider trading legal would make it clearer to individual investors that picking and choosing stocks is a sucker's game, and deter more of them from trying, to their financial benefit.
Keeping it banned creates an illusion of fairness that leaves everyone worse off. Felix Salmon, arguing for bans on insider trading, wrote, "If you want a nation of shareholders, you need to give individuals some faith that they won't get picked off like so many fish at a poker table." But we don't want a nation of shareholders — or we shouldn't, in any case. We want a nation of index fund holders, and banning insider trading makes that harder.