Discussions on this forum are always interesting and a gold mine for a budding sociologist.
On the one hand , we have the risk averse group from across the Pond with the opinions of why it should not be done except in very exceptional circumstances and with appropriate precautions.
The you have the risk tolerant group from the US that says it is not a big deal,been done before, can be done routinely, everyday, all day and twice on Sundays.
In my never to be humble opinion, it is aircraft dependent and activity related. When I flew BO105s and BK117's for EMS, we would keep both engines running at idle at a scene, cyclic lock on ,SAS off and collective frictioned down and locked.But these aircraft were specifically designed for such operations. Matching the aircraft to the operation is key in risk management. An AS350 I believe is not a good candidate for leaving it unattended. Besides, with its great shut down and start up time frame, there is no real need for the average commercial operation.
One last thing, a long time ago an RN pilot said something very profound that has stuck with me all these years and has helped me several times to navigate the cultural minefields:
In the UK and other Commonwealth nations,anything that is not expressly authorized is considered forbidden.
In the US anything that is not expressly forbidden is considered authorized.
Alt3.