I can't think of any particular reason that wouldn't work, unless Voldemort or the Death Eaters in general have some way of stopping them prior to the attack. Maybe the anti-disapparition jinx (which is an area of effect spell rather than a targeted one according to the Potter wiki) also affects portkeys? Or perhaps there's another spell. The Death Eaters might simply jinx their targets' houses before attacking.
If there weren't some way of preventing people from using them, I'd think having a portkey in the house in a readily accessible place, to teleport out of harm's way, would be standard response for anyone at particular risk in the war. Given that this apparently didn't stop the Death Eaters, I assume that it's preventable.
Given how poorly the war was going for the OotF I wouldn't be surprised if by that point they were suffering from a shortage of safe houses.
Edit: New thread posted here.
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 86. The previous thread has long passed 500 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.
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: