It's not always that easy, I've found myself having to go to a place with high-bandwidth internet to fetch stuff, and being able to stay there only a few minutes. Or before boarding a plane(you don't really want to pay for internet on an intercontinental flight), etc...
Also, while you might do things as-needed, it's not that difficult to always remember to run an update every time you run brew. And I wasn't even proposing to make it the default, just to make it possible to run those 3 things separately.
All I'm saying is that you really need to investigate into the things you are suggesting. Please do the investigation, even if cursory before you jump in.
I am guilty of the same, but in my case, I have an installation I can actually check. You can read the source code if you don't have a mac to work with, but the important thing is this already exists
