Nice idea, but I don’t think this would be scalable.
Imagine you want to share an entity with Alice (as an Editor) with Bob (as a Viewer) and with Charles (as a Commenter). It would seem that you would need to set up a field for each possible access level.
And given that there are options for custom access templates, this could become unworkable/painful.
Of course, if relationships had attributes, I could imagine a ‘shared with’ collection field where you can add users, but also specify the nature of the sharing for each user, as a property of the relation.
Unfortunately, I think it’s a long way away…