I'm less worried about the face scanning (you can opt-out, as you said), I'm more worried about the camera scanning in other parts of the airport. The glasses combat the most common form of face scanning, which uses IR illumination. It also works at grocery stores and whatnot, which is especially important if you're a POC and likely to be racially profiled as a shoplifter (I've read some horror stories).

It does paint a bright red target on my chest since they show up as a massive bright light source on IR feeds (if a security guard happens to watch), so it's more a form of protest than anything.