There are some folders that are really used to group/place messages for a short period of time — hence, their nature aren't quite static as the rest.

I think it would be great to have a way to hide them, as long as they contain no messages. Candidates for this category are Spam, Drafts, and Archive — but users do create their own, so it's really hard to tell.

Regardless, this should help to unclutter the interface in case there are too many folders.