You don't understand the main concern of SAG. Extras aren't typically well trained actors. It's either people just starting in acting or.... regular people pulled off the street.
If I'm down on my luck and the studio says $200 for a day, we scan in your face and we use you in movies. I'm going to take that... as will a lot of people. If you're in a movie studio town (like Hollywood) you could even sell your likeness to a whole bunch of studios (since they can't patent your face).
In most movies these days extras are less than just background. But in scenes where you can see faces it's kinda already happening. Movies will take just a hand full of extras and through CGI editing copy and paste their flat images into a scene. In
$1 they grew the size of a crowd by 10x by adding just 8 re-usable extras.
Having a "catalogue of faces" to do this with means people scanned it can be used like stock footage.