Are people good judges of how much value they bring a company? You may think you deserve more pay, but you may also have a high opinion of yourself or over-estimate your skill/contributions/merit.
I think making salaries public information is a double-edged sword. On the one hand it reduces information asymmetry and gives you more negotiating power, but on the other it could probably lead to sour grapes and/or hurt feelings among employees where there was none before.