In short, winching operations should be limited to when there is absolutely no alternative. SAR crews face quite enough "routine" threats in their day to day roles, without facing (knowingly or unknowingly) unnecessary threats.
We don't winch but use a double hook and sling people in and out of situations. One of my crewmen is a sling junky and likes to fly under on the line, and I have to constantly tell him no, and to Carry the patient the small distance to the helicopter (where possible).... no matter how many times I explain the risks of hanging underneath to him...
The few extra minutes don't make much difference to the whole rescue.