In the private sector, it's all justifiable - if you don't justify your salary, you don't keep your job. Simple as that.
Public sector, this becomes a much more interesting question, as you have to balance the salary against the value to society. In this regard you can clearly see some issues with salary/value comparisons (eg. nurses?).
Wouldn't like to guess what the person with the "highest" value to society is..... although I don't like it (and I'll probably get flamed to death for saying it!) the correct answer is probably senior politicians.
To explain - I think the current political process is too heavyweight, beaurocratic, corrupt and expensive, and we probably all think we could do a better job, but I suspect that it's not true when it comes down to it - the good politicians who put the work in and manage to get things done despite the process are probably the pinnacle of true value to society. Personally - I believe that the top politicians are probably underpaid in truth. They earn many, many time less than their private sector counterparts - I think it would be fascinating if characters such as Richard Branson and Willie Walsh were in politics..... but I don't think they'd accept the massive pay cut!