I can tell you i've spent countless hours staring into mirrors attached to airplanes trying to see a bogey on my short 6...
what i'm talking about is glider towing
But i can tell you .. if it's not for engine vibrations, air buffeting, turbulenze, etc will do the trick , i mean the field of view is VERY narrow and seeing a glider attached to your tail with a 30 yard rope can be dificult enough sometimes, i don't even dare to beliefe they were of any use to see a bogey on your long 6 saddling up for the kill ..
IMHO they were more used for formation flying and ground operations (yes it might be important what's behind you on the ground) than actually in combat (even tho i think it WAS possible to see guys like Hartmann in your mirror (usual kill distance under 50 yards) but i guess if you saw him you were dead anyaways

and here's a pic to comprehend.. th "target" is a LO100 aerobatic glider wingspan roughly 30 feet)
DW6