I've never thought of it as a method of achieving a graduated form of response. It's more a matter of priority in communications and traffic handling. Thus, a call of "PAN" will take precedence over someone else who has just provided a position report, or an enroute weather report, etc.
If someone then calls "MAYDAY", that communication gets the priority over PAN calls and routine communications.
The same sort of priority should apply to aircraft approaching an airport for landing. In the instance where a PAN call is made, that aircraft gets priority for landing over everyone else... unless someone else turns up screamin MAYDAY. Well, that's how I've always understood it and, notwithstanding the hard won experience of those claiming that PAN is a useless call in some parts of the world, I believe it has a clear implication for safety of flight.
That was why I suggested that some action needs to be taken in regard to the countries where "PAN" is unknown. But, then, I guess that if you've got the sort of trouble that requires a PAN call in a country like that, you've surely got even bigger problems than a lack of knowledge of PAN...
