If mirrors can be fitted, they are an extra piece of the safety jig saw puzzle, just to increase the situational awareness of pilot.
Once had to go to a court hearing, B206 went down due to sling being attached when not supposed to be - mirror was broken and not fitted. Pilot did not see the sling was hooked up - though it was in the back with a passenger who had climbed aboard (for the last time!).
B206 took off, sling caught on an obstacle and broke. Flipped back up into rotor, taking off about a metre of the blade at the tip.
At 70ft the main rotor blade came into cabin and sliced the pilot into two!
Upper and lower body were tossed in different directions - 130 metres. A terrible accident which killed others as well.
Pilot had been used to mirror, but on this day........