>the other states that this discourages non-insider trading, which actually makes the prices worse

This theory is fundamentally not credible, the other side of any trade you make on the stock market is essentially always going to be vastly more sophisticated than you. Insider trading makes zero difference to the end-user.

The credible argument against insider trading is that it's a form of theft. You are making trades based on information which does not belong to you, and which you have an obvious duty to protect. You are essentially stealing from the people you work for.