Sorry. In any circumstancxes at all, A MOB is a PAN PAN call. Mayday is of direct and immininent threat to the VESSEL. Don't worry, hearing Pan Pan, Pan Pan, Pan Pan over the airwaves still gets you all the help you are ever going to get with a mayday.
If you need to conduct peculiar manuevering in a situation where others are likely to need to dodge you, then a securite call is indeed appropriate.
Remember the devisions.
Securite calls are low level notices of interest to feelow mariners regarding weather, hazzards to navigation and such. Securite calls are about making announcements, not requesting assistance.
Pan Pan calls indicate a danger or or crises beseting your crew or vessel that has not eyt put the vessel in iminant peril. Crew members becoming ill and requiring assistance, finding no way to start your motor (presumably on a day too flat to sail in) when you go to pull up anchor or (in our own case) having the stern gland implode and beging flooding the boat with water (however I believed that I could manage the situation so placed an advisory pan pan call to the CG, advising them that I wanted to handle it and retain control of my vessel, but that the situation might deteriorate quickly and I would appreciate them keeping an ear out for us).
Mayday call. Vessel in immediate distress. At this point you are pretty much relinquishing command of your vessel to your rescuers. The stament is that you are willing to abandon ship and are asking to be taken off.
The reason that "in the odlen days" Mayday calls were so much more likely to get a response then the lesser Pan pan calls was that the salvage boats would also come out. So you were getting the attention of people that wanted to pirate...errr...salvage your vessel as soon as your feet left the deck and make some money off the deal as well as the good old Coat Guard. This is not so much of an issue nowadays, nor for our class of vessels, nor in 99% the waters we sail...so do not feel the need to overuse it.
P.S I have now done two "for deadly real" COB rescues. One when I was the only pone left on the boat in really crappy conditions and when when I was part of a larger crew and somoene did something stupid in fairly calm and controlled bay waters (albeit in a narrow channel, with heavy traffic and bad shallows on either side restricting manuevering). Not the most fun thing in the world. Having time for radio calls of any sort in the first instance was completely out of the question. In the second instance, with the 8man crew, there was little need.