I don't really see how that's different from a normal binary install of a reproducible package. Especially with the lacking quality of a lot of Nix packages.

If you're in a situation where you want reproducibility you're using nix to build your own packages anyways, not relying on their packages

It's not if you can pin the package. It gives you reproducable docker containers without having to rebuild the world. Wasn't that the entire question?