As a personal view, I think that's a very bad idea. Even though most things are stable every few upgrades you see will something from some small conflict to a full-scale meltdown. It just happens when there's so many plugins and themes that can be used in a single system. If the system did automatic updates for you, there's a chance that you'd never see these issues until you start getting complaints from users - or even worse, users stop visiting your site because it's broken. I'd never want to have updates applied ot my site without me knowing and doing a full (files + DB) backup before I start upgrading anything.