Your car has a mirror why not just take it out you can just look back.
Why is the mirror there just to check your lipstick?
The human eye is attracted to movement the mirror looks back 100% of the time. Even a dot moving in the mirror against a blue sky will attract your attention. It adds another level of SA.
The addition of a mirror if done correctly will not affect frame rates on most of the machines today; being the area displayed is very small. Planes in DCS have the mirror with the option to turn it on or off. On the machine I am using it causes no loss in frame rate. I use it mostly on ground attack missions in the P51 to confirm hits on ground targets, and to stop those pesky 109’s and 190’s from trying to sneak up from behind.
If the WWII plane had them then they should be there and the choice should be up to the pilot whether to use them or not.