I understand their rationale for it, but I don't think it should be implemented. Having something immutable like this allows it to end up being used for tracking purposes. Just add a <script src="trackingcookie.js" /> that calls a function with the cookie and all the sudden there's yet another place to covertly store an ID for tracking a user.
But you're removing HTTP requests, so how can it make tracking easier? Any ID that a company puts in their immutable content can also be put in their normal content. The change doesn't make tracking any easier than it already is.
It makes it another place that they can store the tracking cookie and have your browser give it back out to them. Deleting the cookies or other such things wouldn't remove the tracking ID as long as the browser continues to use the cached immutable resource. Similar to how this[1] works by using multiple storage methods.
