I don't know whether there's any reason they'd have been using Parceltongue.
In general? Security - same reason morphed Quirrel and Harry use Parseltongue. That's why they are in the forest in the first place.
I believe that you are overthinking this. Look at other writings, that have nothing to do with snakes, and you will find examples such as "'How dare you speak to me that way!' she hissed" as a way of speaking when angry. I seriously doubt that it has anything to do with Parseltongue.
No, I meant Quirrell. Could Quirrell understand Parseltongue without being able to speak it?
Yes, because Dumbledore canonically could.
edit: Here's the link: Just search for 'Parseltongue'
Update: Discussion has moved on to a new thread.
The hiatus is over with today's publication of chapter 73, and the previous thread is approaching the 500-comment threshold, so let's start a new Harry Potter and the Methods of Rationality discussion thread. This is the place to discuss Eliezer Yudkowsky's Harry Potter fanfic and anything related to it.
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. The fanfiction.net author page is the central location for information about updates and links to HPMOR-related goodies, and AdeleneDawner has kept an archive of Author's Notes.
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: