Because there is no way Voldemort would be able to cast the true patronus. The nessesary mindset is much to altrusitic. Hmm. Since Hermonie could likely also master the true patronus, wrecking the animal form of the spell for everyone probably shields her from excessive punishment in general as well, at least until such time as a significant number of people other than her and Harry master it. The optimal version of this play is to have Hermonie destroy the dementor present´, but that requires enough time to prep H. Timeturner?
The nessesary mindset is much to altrusitic
I'm not even completely sure that's true. Maybe Voldemort could cast it with a sufficiently strong-willed rejection of death for himself (and for his allies), and out of hatred of dementors=death, if he knew how.
EDIT: New discussion thread here.
This is a new thread to discuss Eliezer Yudkowsky's Harry Potter and the Methods of Rationality and anything related to it. With two chapters recently the previous thread has very quickly reached 500 comments. The latest chapter as of 17th March 2012 is Ch. 79.
There is now 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.)
The first 5 discussion threads are on the main page under the harry_potter tag. Threads 6 and on (including this one) are in the discussion section using its separate tag system. Also: one, two, three, four, five, six, seven, eight, nine, ten.
As a reminder, it's often useful to start your comment by indicating which chapter you are commenting on.
Spoiler Warning: this thread is full of spoilers. With few exceptions, spoilers for MOR and canon are fair game to post, without warning or rot13. More specifically: