I think I came across different than what I intended. I personally think it should all be out in the open, but if the employer has a policy against discussing compensation, and someone else comes up to your boss and says "Why am I not getting paid as much as Michael", then it is quite likely that your boss will try to rectify this situation (squeaky wheel and all). And since he is only given a fixed amount of money to distribute in raises, it is likely that yours will be affected negatively by that.

Now if you feel that the other employee is being under compensated, and you are getting too much, then that may work out. Or, it may be better to work somewhere else that has more money to spend on employee salaries. But the point still stands, that just because something is legal for you to do doesn't make it a good idea to exercise your rights in every situation.