Mirrors were initially fitted on British registered SA330 Pumas as a means of checking the build up on the snow dam in front of the engines. It was also invaluable for checking the physical situation around the engine cowlings in case of fire warnings, loose cowls etc. When the 332 Super Puma came along with their different intakes management and engineers decided that mirrors were unnecessary.
Then Doogal (RIP) had a double engine failure owing to ice accretion on both engines. Fortunately is was at several thousand feet so he had time to get them restarted. All the aircraft then had mirrors within a week.
I flew for a long time on an operation in China where there was a mix between British and Chinese registered aircraft. The G reg with and the B reg without. Personally, I was happier with mirrors. I had to use them in anger several times with regard to EFWs which helps to keep the heartbeat rate down. Conversely I never had a fire warning on one without mirrors but they were still reassuring if you had some strange rattle around the roof area.
I noticed that Bristow Australia still retained them for their operations when I flew for them.