In Eliezer's HPverse, that may be a sensible argument. (Given the general irrationality of wizard-dom, not a very strong one, though.) I'm criticizing Eliezer for diverging from canon, which IIRC has no suggestion that armor would be useful or had been tried but abandoned in the past. (The only example I can think of is maybe canon had goblin armor, and I'm not sure how that would apply.)
Canon already suggests spells can be stopped by solid objects, but only if they're sufficiently solid. And powerful spells have been shown to blast objects, while weak spells haven't. It's not much of a leap. In HP canon, historical wizards may or may not have worn armor of some sort, but for an adult wizard, armor is probably more trouble than it's worth. Considering how versatile a properly trained wizard can be in combat, it shouldn't be able to do more than force the opponents to slightly revise their tactics, while increasing the wearer's fatigue.
Rem...
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: