I agree, great explanation! (although I did have to read it twice this evening… Cold dr ugs are winning I’m afraid…)
I think an EASIER explanation (IMHO) goes along with Petr’s sentence about OSPF being a “leaf” of a larger setup, or back to how it was originally created with dial-up ISP’s gathering routes from each other.
You MAY get a default route from one of those connections (but most likely not back in the day), but you’d get appropriate summaries for the connected AS’s routes.
In other stub areas, the assumption is the focus is your OWN routes, and therefore since you lose visibility to other external (type 5) routes, you would want to propogate that information. With the advent of NSSA, that was no longer the assumed focus. At least given the state of network usage at the time of its creation.
Now-days it seems more a nuisance than anything else! 
But as usual, Petr is spot on!
Scott
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