Why does Quirrell want Harry exposed to Dementors?
At the risk of building this theory on top of another unconfirmed theory... It's been speculated that Quirrell himself is Demented. He doesn't appear so when Voldemort is telepathically controlling him, but when Voldy takes a cigarette break or whatever Quirrell enters zombie mode. Quirrell is just kind of an empty body, zombie-like unless Voldemort is logged in.
Maybe Voldemort wants to control Harry's body in a similar fashion. What the difference is between dementing and then telepathically inhabiting, versus simply using the Imperius Curse... /shrug.
[Update: and now there's a fifth discussion thread, which you should probably use in preference to this one. Later update: and a sixth -- in the discussion section, which is where these threads are living for now on. Also: tag for HP threads in the main section, and tag for HP threads in the discussion section.]
The third discussion thread is above 500 comments now, just like the others, so it's time for a new one. Predecessors: one, two, three. For anyone who's been on Mars and doesn't know what this is about: it's Eliezer's remarkable Harry Potter fanfic.
Spoiler warning and helpful suggestion (copied from those in the earlier threads):
Spoiler Warning: this thread contains unrot13'd spoilers for Harry Potter and the Methods of Rationality up to the current chapter and for the original Harry Potter series. Please continue to use rot13 for spoilers to other works of fiction, or if you have insider knowledge of future chapters of Harry Potter and the Methods of Rationality.
A suggestion: mention at the top of your comment which chapter you're commenting on, or what chapter you're up to, so that people can understand the context of your comment even after more chapters have been posted. This can also help people avoid reading spoilers for a new chapter before they realize that there is a new chapter.