Site environments are far too varied for forced automatic updates to be particularly viable, but I think as an opt-in feature it could be very useful. For instance: due to the git workflow I use to develop and deploy most sites, it would be a nuisance for plugins to start updating themselves on the production sites - even if they are non-breaking updates. As well, there's a lot of room for abusing such a feature (think a bait and switch being silently pushed to sites with the previous plugin).