[derefr]

Sounds convenient for one major use: letting web-scraping and readability-like tools know that everything outside of <main> is alright to throw away.

[pestaa]

I consider navigation not alright to throw away, but wouldn't put it in <main> either.

[wtetzner]

For something like Readability, you don't want to keep the navigation. The whole point is to get just the content, and make it readable.

[oneeyedpigeon]

But if I'm using a readability tool to access your content, I certainly don't want the navigation to be a part of it.