I'd argue that almost 100% of users, myself included, can't really make this analysis for any given update. Usually you can't anticipate what will brake, that's

I'd argue that almost 100% of users, myself included, can't really make this analysis for any given update. Usually you can't anticipate what will brake, that's what makes upgrades scary. What everybody does however is to use past experiences to evaluate the risks. Who hasn't had a system upgrade break something that took a while to fix? In these conditions, who wants their system to auto-update if the system is critical and it could happen at the worst possible moment? As I said I try to keep my system up to date, but if I know that I have an important deadline in the near future I'm likely to postpone updates to avoid shooting myself in the foot.