The big issue with using twins is largely that (in Europe at least) you need a twin to have IFR capability. You can't go IFR (and shouldn't go IMC) in a single. End of story. And no-one here sees that changing. I had thought that applied in the US too, but maybe not? I'm sure someone will confirm. Apart from engine redundancy, an IFR twin will also have redundancy of many other systems such as instrumentation, hydraulics, generators and inverters, providing added safety when stuck up in the clag for some time.
As it happens, I'd be fairly comfortable in IMC in a well maintained and preferably newish single like a 350B2/3, with a good autopilot, but the law doesn't allow it in Europe. Having said that, I'd still take the psychological comfort (as would most pax) and some practical benefit of a second engine!
SASless; you say the pilot/operator should make the decisions not the government. Ideally yes, but history proves that pilots' judgement is too often not adequate. Legislation therefore reduces the scope for such decisions - and getting them wrong. They can also take pressure off a pilot by enabling him to simply say, for example, I'm not legally permitted to take the B206 on the flight tonight. It will need to be IFR twin or not at all. Don't get me wrong, the extent of legislation can be very frustrating, but it does have its benefits.
And finally - as I've asked before with no response - does anyone know of any safety statistics relating to European and US EMS ops? Clearly they need to be interpreted carefully and can be misleading, but would be useful.