I think this is the bit relevant to my question -
When I bought the radio I use in the US, I specifically chose a waterproof DSC model I could wear, and I did so because I knew (a) the Coastguard would be monitoring it, and ( if I had to use it, I would almost certainly be on my own and in the water, which means that making a mayday call may not necessarily be straightforward.
In NZ, however, knowing that Maritime Radio won't receive my DSC call and with at best an indeterminate number of private DSC-capable radios in use, for me DSC offers little assurance that it will be a benefit when I need it - and the comment in your article about the commercial fisherman kind of underscores the point. So when I'm in NZ I see a PLB as a far more important safety measure. I bought one in the US to bring to NZ specifically for that reason.
I think the question is - who do you want to contact and who are you contacting?
If you have someone aboard who is having a heart attack, fallen overboard, you're sinking etc - you should be wanting to contact as many people as possible, especially those closest to you - remember the coasguard is probably 40 mins away unless they're patrolling in the area. So how do you do this? Lets say that there's 6 boats within an effective range of your position to help. I would guess only one is going to be monitoring CH16 effectively. A couple of them will be on the local coastguard frequency but likely the volume will be turned down since everyone is doing trip reports. The remaining will be on ch6 or weather channels etc.
If you have a standard vhf - you make your mayday call on CH16 - this is heard by Maritime radio who will take control of the situation - your local coastguard will transmit on their frequency as well - and start to get their boat organised - call in crew etc. Of the 6 boats within effective range - the one monitoring 16, as well as any who are on the local coastguard frequency with the volume up will be notified of your distress.
If you have a DSC VHF - you push the button and make a mayday call on CH16, Maritime radio, local coastguard will respond as above. In addition any DSC radios correctly set up will alarm and change themselves to ch16 - so more people listening who will be within effective range.
I should also stress - just pushing the button and crossing your fingers is not going to do any good. It is ALWAYS to be used in conjunction with a mayday call on CH16. Also - it is not to be used as an alternative to a PLB / EPIRB - but again, these do a very different task.
The commercial fisherman in the article was at the dock the whole time, I should have noted that in the article - hence he didn't hear our calls on CH16 after the test to advise that it was just a test.