As the one who wrote the TODO comment originally, I think it's fine to cache them. I originally disabled it to avoid poisoning my own cache when developing in case the action needs to be reworked but now that it is merged and works I think it is fine to enable.

That said I don't expect much caching to happen, as the lock action takes into account the commit hash of the opam-repos, thus they do get invalidated rather quickly. This might end up filling the cache with single-use lockdirs that rarely if ever ever generate any cache-hits.