Agreed, when it comes to collections, I don't see how immutable data can be used. However, this begs the question: what use cases will immutable data have in most applications? All data in an application needs to be predictably reference-able from somewhere, otherwise it'll never be found. Whether we're talking about a list of subreddits, threads, comments, likes, donation lists, whatever, since everyone "shares" access to SD objects, I don't currently see how anything else can be used; immutable objects aren't share the same way.
This also begs the question: why not have SD that is both immutable and mutable? If a tag_type + identifier is set as immutable, it will cost less (same as the current immutable data), and if mutable, make it 10x more. That way both types of objects would be shared, and whether it's immutable or not would depend on its usage.
I'm trying to fill in the gaps of my understanding to the best of my ability in order to build out the JS library. The idea is to abstract as much of the technical aspects away as possible and make it easy for the average dev to use.