ATP IFR.
Your point about proficiency waning is right on. I read a study where there was little correlation between being instrument rated and not being instrument rated in the performance of an Inadvertent IMC excursion exercise if skills were allowed to diminish.
On the other hand there was positive correlation between 1. Having an ATP rating 2. Maintaining Instrument Skills and 3. Passing an Instrument checkride and good inadvertent IMC skills.
Almost all of our mission flights are conducted under VFR. We occasionally evacuate for hurricanes and conduct admin flights IFR (with 2 pilots). In addition, to maintain currency we do some actual instrument flying usually under "soft core" conditions for training and two of us regularly fly airplanes IFR. All pilots take an IPC each year and are required to maintain instrument currency the rest of the time. Our simulated instrument flight is as realistic as I can make it, usually executed at night and over either the Pamlico Sound or Atlanic Ocean so "peeking" is of no assistance in that black over black environment.
It costs a bit in blade hours and dollars to accomplish this extra training but I view inadvertent IMC followed by CFIT as enemy number 1 and I believe it is very worth our while. My intention is the new pilot in achieving his ATP will put a fine edge on his skills...and subsequent training and flight checks will keep it there.
My own thought about conducting air ambulance flights at night in a coastal environment is that regardless of minimums, weather information systems, etc....inadvertent IMC is a when not an if...and the single best defense is instrument competency.
Sorry I was so long winded.