The ActionRegion tag is more useful that what I had thought for many years!
The documentation of this tag explains how this tag defines an "area of a Visualforce page that demarcates which components should be processed by the Force.com server when an AJAX request is generated." So I always thought "If you are asking a question to the user, why do you not listen to the full answer". You are asking for 10 fields, but you only want to listen to 5 answers... That does not make sense! Just ask for 5 fields.
But where I was missing the picture, is that this is not for the final answer, but for partial answers... Let me explain.
This form allows you to create opportunity records with 3 required fields, but when the stage field changes, the time at the bottom should be updated.