I was upgrading some routine stuff on my MacBook today when I got this message.
Yikes.
I think that messages like this are what scare people. They imply, whispering in your ear, "If you mess this up you can ruin your computer. Have a nice day."
I think that software should be made in such a way that is really hard to mess up. For example detect if the power cord is in and refuse to update until that requirement is met.
It's time for some friendlier, gentler software.