Not sure why we don’t see them used more. For designs that are minimizing soldering work, they’re “one more thing to learn to solder”, and there’s lots of folks mimicking designs they see using the standard “standoffs with screws on each end” that it perpetuates.
For the upcoming LDSA keycaps that have lower skirts, you need to have a “clean” plate or you get the skirt striking the screw head. I imagine that might motivate more folks to explore this approach.
Here’s a render of the keymap, you can see the file over at https://github.com/petejohanson/revxlp-config/blob/main/config/boards/shields/revxlp/revxlp.keymap