Probably for any case where an actual human is doing it. On an image you obviously want to do it at bake time, so I feel default off with a flag would have been a better design decision for pip.

I just read the thread and use Python, I can't comment on the % speedup attributed to uv that comes from this optimization.

Images are a good example where doing it at install-time is probably the best yeah, since every run of the image starts 'fresh', losing the compilation which happened last time the image got started.

If it was a optional toggle it would probably become best practice to activate compilation in dockerfiles.