I think that some styles of coif may have ventails that were much bigger than we usually expect - giving two layers of mail over most of the face. With just a thin layer of linen or leather between the mail and the face (at least the part not covered by an arming cap and collar), gices surprisingly good protectoin. Here's my interpretation (which I've shown on this forum before):
http://dawnofchivalry.wikispaces.com/Mail+Coif+-+Biro%27s+project
I like the idea that seperate coifs may have been more common than we think due to 'tucking them in', and the reasoning makes sense to me. I also like the thought that the move to seperate coifs may have partially been to provide an additional layer of mail on the upper chest - if you look at late 13c images of knights charging with a lance, you see them leaning forward slightly - with the head ducked forward. So it stands to reason that any incoming strikes to the face are going to glance off the helm downwards - and land in the upper part of the chest. I'd certainly want two layers there!