I'm sure a TCM/police observer with 20 years experience would be equally capable at spotting wires, and very comfortable with other cockpit duties, however in the UK many HEMS/AA units take their doctors & paramedics on secondments from local ambulance services which offer much shorter service periods, and don't allow people to reach this level of aviation awareness before being replaced. No disrespect to the people who are in this situation but the learning curve is steep, and they always have their medical considerations to prioritise. Two pilots offers a dedicated aviation aware team at the front, and an extra pair of eyes if carrying a crew of 4 instead of 3.