swh:
Mayhap you're right about the frequency of mercy flight declarations, but I cannot recall the last time I heard of one and I'm in the aeromed game.
The criteria are quite proscriptive and any serious operator would likely have much better alternatives for such mundane issues as F&Ds et al - like standby crews. The reality of aeromed is that it is seldom if ever truely 'life saving'; so it is seldom or never that aeromed needs to be in a 'mercy flight' situation.
Can't make any definitive statements on the subject in any area but mine; this is only what I see from my small perch in a big country.
Cheers.