This could be very interesting and exciting. Canonical has confirmed they're working towards an immutable version of Ubuntu for desktop users based on all the work they've done with Ubuntu Core.
I am not sure I understand the hype over the immutable distros. I get that it may be important for large deployments (e.g. a company of 500 people starting with the exact same desktop and remain compatible) but don't really get the benefit for the average GNU/Linux user, especially those of us that like tinkering with our PC. I can't see myself installing one of those but perhaps I am missing the point.
I think you got it exactly right, apart from the fact that an "average" desktop Linux user is like 30% [1] company/university computer. For those users it's probably very useful.
Also it's useful for your average Joe. Joe don't want to mess with his system, so immutability is his additional layer of security. It's not thing for tinkerers, it's meant to be same base on every system, and everything else is container.