Email isn't a completely unused field, though it looks like they only provide publically available emails rather than any ones you're privy to as a connection.
I downloaded my 216 connections and had 1 email address (a chronic startup founder, so he wants to be seen) and 1 completely empty line other than connection date. I just reused that field as one for describing, manually, how I know them since for some awful reason LinkedIn removed the ability to tag people.
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