I hadn't realized that there was such an easy way to bring back lost memories. More reason to put that emerald ring into the Mirror, ASAP.
I took it to mean that Dumbledore had used a reversible memory charm on Narcissa, knowing they might want her back once Harry had defeated Voldemort, or, more practically, they could use her as a "last weapon to be used against House Malfoy" by holding her hostage.
I think Obliviate is still permanent for all practical purposes, so the same couldn't be done for Voldemort.
[edited iteratively]
This is a new thread to discuss Eliezer Yudkowsky’s Harry Potter and the Methods of Rationality and anything related to it. This thread is intended for discussing chapter 120.
Plans for next chapter release:
There is a site dedicated to the story at hpmor.com, which is now the place to go to find the authors notes and all sorts of other goodies. AdeleneDawner has kept an archive of Author’s Notes. (This goes up to the notes for chapter 76, and is now not updating. The authors notes from chapter 77 onwards are on hpmor.com.)