Here is another argument for the sake of the company:
If insider trades are legal, investors might read too much into certain such trades. Board members sell shares all the time. But because insider trading is illegal, we kind of know that they are doing it for personal reasons (bigger house, divorce, etc) There would be a constant down pressure on the stock because the insiders keep selling their shares for legit reasons.
It would be also easier to manipulate stocks, "he must know something, he is buying."