From what I can think there will be two broad use cases:
1) Sibling pages within a section will often share RHS and header content. So makes sense to replace the inner area where the actual page content lives and leave the rest alone.
2) Cases where the above doesn't hold or it's too complicated to care about, so load in the whole body.
I'm sure DHH chose the #2 rule because #1 turns out to cause a can of worms?
And the inevitable vs PJAX conversation (ie: is it flexible enough? Does it need to be?)