The fact that there a troll in Hogwarts is a sign that Hogwarts is under attack by someone who thinks that he has something to gain from smuggling a troll into hogwarts.
If you notice that someone breaks into your house, you call the police. It's the right thing to do even if you sit with a bunch of friends and you have a bunch of riffles at home because you are an American who loves the second Amendment. The population of magical Britian is very small so Bones basically heads the local police department.
McGonnegal thinks that it's plausible that the troll is a distraction to an attack on the third corridor where something imported get's stored.
There's fog of war.
Even in the case where Amelie Bones has some other fight to fight at the same time, McGonnegal traveling 6 hours back and time and giving more information to Amelie Bones, helps Amelie Bones to understand that multiple targets in magicial Britian are under attack at the same time.
If Hogwarts staff can't handle that, they're not worthy of their titles.
Evidently they can't handle it and as a result one student died. Even if they were 95% certain that they could handle the troll on their own, it would still be prudent to go back in time and place a call for help.
Don't forget that there are people outside of the Great Hall like the Librarian.
If you believe this is a distraction, why on earth would you use your ace in the hole dealing with it? Wait to see what it's distracting you from, and then start burning resources. Unless you want Bones to be out of time-turns when she finds out that she needs to go back an extra ten minutes.
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 90. The previous thread has passed 750 comments.
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: 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17,18,19.
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: