It says early in the chapter, when Harry and Neville are alone, that this didn't count as giving Voldemort a good idea b/c the armor would only stop minor jinxes.
But this is for the crappy armor that first years can both build in a short period and also wear. A full grown adult with governmental resources ought to be able to obtain and wear much better armor.
Given the problem Aurors seem to have with surprise attacks, that alone might make them worthwhile!
(In the real world, no one says bulletproof vests can stop only weaker bullets and don't do anything about explosions or knives, so there's no point in equipping soldiers or cops with such vests...)
Update: Discussion has moved on to a new thread.
The load more comments links are getting annoying (at least if you're not logged in), so it's time for a new Harry Potter and the Methods of Rationality discussion thread. We're also approaching the traditional 500-comment mark, but I think that hidden comments provide more appropriate joints to carve these threads at. So as of chapter 67, this is the place to share your thoughts about Eliezer Yudkowsky's Harry Potter fanfic.
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. The fanfiction.net author page is the central author-controlled HPMOR clearinghouse with links to the RSS feed, pdf version, TV Tropes pages, fan art, and more, 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: