I often use different email addresses in order to track spam sources, especially if I am not very sure of a site before I sign up to it. (For instance, because of the reputable lineage of this site, I did not create it its own unique email address but instead reused one, but I am using a “forum” address that I often use at new forums or anything that I may want to tweet, rather than my “main” address. That was simply reflex! Some forums don’t feel “forumy” to me so I use my main address.)
Also, email addresses change quite often, sometimes abruptly, particularly if users are signing on from work (jobs change quite a bit for most people over their course of their life), hosting sites change or vanish, or as users abandon or change email address for whatever reason.
I am also not clear that all of your authentication services use an email address as the key or if they do if they will share it with you – twitter keys off username, for instance, and while G+ is tied to a gmail account, I believe that their own account merge functions mean that it’s really their account number that is the real user key, because that gmail account can change over time.
Some way to merge two accounts, even if their email differs, would support a wider range of activity and a longevity of presence and a good ability to consolidate identity even if the email “key” changes with time. You may also want to think about secure ways for users who are no longer using an email account to regain access to that identity. It could be as simple as being able to add backup authentication (such as using a second email account or facebook or twitter) and hence already part of a sensible merge function.
Reputation systems only become stronger if the code implicitly encourages users to link an active presence on a forum to an active presence on another forum, without forcing them to do it and letting them try it out first to see what it’s like; abandoning those “throwaways” that become real accounts later seems like a waste.
Also note that if you permit it, non-Discourse services might start using you as an identity service in your own right (I could see it being a rich identity service), and plan accordingly for a way for people to be able to organize a durable identity that spans multiple email addresses, changes of name, and phases of life.
You may want to key off a more durable-to-you identifier than an email address, such as unique username or unique username(at)discourse.org. (I.e. someone who wishes to be foo on the playtime.org forum and baz on the workstuff.com forum yet logs in to both of those using a common email address of biff(at)gmail.com may not want to “cross the streams” and have those two identities automatically consolidated even though they are using the same email address.
If you do wish to consolidate via email address, then make sure the user knows it will happen so that there is not an inadvertant “outing” of foo(at)playtime.org to their colleagues at workstuff.com for their perfidious nerf habits.
Even with sophisticated users, identity is just highly fluid, for a lot of reasons, and it’s not necessarily a bad thing to let people start out compartmentalized and tentative and consolidate later when they trust the site.
(Sorry about the clunky examples – I am brand new here and have not figured out how to escape the ‘at’ sign for example email addresses.)