I think these headers only make sense for browsers, so excluding webdav makes sense I think.

The reasons I could see to go for either of them:

• If we want any unauthenticated requests to do something on a specific collection, then including the email in the path makes sense
• For sharing purposes it may make sense to be able to request another users collection

AFAIK sharing is handled differently in DAV (shared folders are represented within your own collections), so /dav/files seems ok?
Could there be a "public" share over webdav that benefits from using the email address in the path?

I don't think we have to keep backwards compatibility though.