You raise a much discussed point. My personal belief is we should call it Single Pilot for ops with only a single pilot on board as crew, we should call it "Multi crew" for ops with a single pilot plus additional non pilot crew, and call it Dual Pilot for ops with more than one pilot. Ah...semantics. It does save emotive responses though!
I think the crew thing is a cultural issue that needs to be considered within the context of the particular organisation and country. We conducted a study tour through the US and Europe looking at various HEMS organisations (and I mean EMS and SAR when I say HEMS). We concluded that the most appropriate mix for HEMS was a Multicrew solution (1 pilot plus at least 1 crewmember). Further, our view of "world's best practice" is based on the newer EASA system which states that there should be two pilots up front for all HEMS missions (day or night or NVG), however a non pilot crewmember can fulfil the second pilot role if appropriately trained.
Norweigan Air Ambulance (NOLAS) have put into place what we considered to be the best method of meeting that: their crewmen are trained at least to PPL helicopter theory and most often higher. They are given the flight training to land the helicopter, they are given an extensive left seat course including IFR and NVG modules that extends to almost 40 hours by the time they are fully qualified.
We use the crewman upfront for transit and enroute work, and down the back for hoist or tight confined area ops. Our CAO states that the second crewman be positioned by the PIC, on intercom and proivide assistance as required by the PIC and Ops Manual. Each organisation in Oz trains the crewies to different requirements, some do not have a front seat element to the training at all, others do a full IFR course.
Over in the US they are having a big debate because the NVG advisory tells them that the second crewmember (required for ops below 500ft and to unprep HLS) needs to do the full NVG ground school. It does not specify any flight competencies, nor even the equipment they need to be using in the helo. Accordingly, many organisations use the paramedics and give them the ground school and equip them with a NVG monocle and meet the specs that way. They are yet to have an accident in 15 years of NVG ops. not wrong - just different to our way.
The Swiss have 25 years and they require the second crewmember to be unaided!!! The viz limit is when the unaided crewie cannot determine if the acft is moving or not. They traditionally dont do many NVG landings. Again, not wrong - just different to our way. Again: accident free for more than 25 years of NVG rescues and SAR.
So it will fall to each country to decide what fits within their risk profile and operation and culture. You Poms can get a really good priced airfare over to Oslo and check out NOLAS for yourself: I highly recommend it. Happy to show you around both our rapid response system and our SAR operation, but Oz is a loooong way.