Sure, I’d be happy to share my thoughts.
From the admin side I think it would function the same way. Essentially it would trigger a “something else” type flag PM where the selected part of the flagged post would appear in the PM.
I would not say that it happens often, just that flags on giant posts can be hard to parse through. Flagging part of a two sentence post wouldn’t be particularly useful, but flagging a part of a large post (for example my Discourse Moderation Guide) would be extremely useful.