I doubt Facebook or any third parties will find much benefit to this. They will just say that they will protect their own CDN. Otherwise they effectively kill their ability to update their scripts. In fact, I'd expect them to deliberately disable this by updating the script with a random byte every so often.
If a site wants to pin a third party to a specific version, they'll need to copy the file themselves. Though I'm not sure if this can be detected and "fixed" by the script author. I've noticed that Stripe's js file logs a warning if it thinks it's being loaded from another domain.
