You're not just talking online games, you're talking enormous-world MMOs, and that's such a far cry from the areas that I have worked with and thought about how to work with in any detail that I can't usefully add anything.

If you're trying to manage the concurrent state of 10s of thousands of players in a game, all server-side, and you want specific actors to handle that single player and you don't want a globally persistent state, then I suppose this makes sense.

I've never worked with anything of that scale though.