I haven’t found a reason to use them that outweighs the time spent trying to keep them in place or locating them after it falls off! Lol…
I have heard to use it to keep from being lit up behind the optic. I’d say they probably need to turn down the brightness a little. Secondly, I keep my rifle locked in the tripod, muzzle pointed down, and in sleep mode until I find something of interest while scanning with my handheld unit. I don’t go live on the scope until I’m behind it.
I haven’t run into a situation yet, where I could find an advantage to having the eye cup in place. Possibly if you’re using it for daytime hunting purposes I can see it having a use, but otherwise, I don’t bother with it.
To each their own though. Necessity is the mother of invention! If you really want it, and get tired of replacing it, come up with a way to keep it in place. Depends on the way it’s mounted of course. Maybe a Zip-Tie, fishing line, dental floss or maybe even rubber cement or some other adhesive?
Good luck!