I think the questions you ask could be directed at any aviation operation, when it comes to it the decision making process in arriving at whether you winch or not has to be taken case by case and those involved in Police/HEMS work I'm sure appreciate the extra risk. Alot of other countries already have winches fitted to their Police/HEMS aircraft and operate perfectly well with them, alongside conventional SAR not instead of.
With a good initial, continuation training regime and robust SOPs winching could be just as a useful capability with a Police or Air Ambulance operator and their decision to use it I'm sure would not be entered into any more lightly than a military or civilain SAR unit, I suppose its the knowing of ones limits and that applies whether you have a winch fitted or not.