It'd be useful to find the effective user for a given email address. The current potential use case I have in mind for this is to detect Phabricator users using a given (already public!) email address, and subsequently use data attached to that account (e.g. in the case of Wikimedia Phabricator, find out someone's IRC nick, as that's the only place where an IRC nick is stored). A quick look at the Conduit APIs doesn't say much about something for searching/querying identities.

In general, it'd also be good to have an email constraint for user.search based on whether someone wants an email to be "set public" or not, though that's a more complicated task for later.