Ok, let me try to rephrase. I guess it boils down to fundamental concepts in wordpress mu internals/concepts, about which I know just about nothing, so am trying to learn.
My impression is, the user login is the key to everything. NOT the email address, and NOT the combination. That is, if my login is 'joe', it fundamentally doesn't matter *what* my email address is. Now, I know that the system currently *does* care about this when adding a user - the add-user code checks to make sure the email address is unique, and if not, you get an error. But is this the *only* place where it matters? If you had two logins with the same email address, would the system break somewhere?
Yes, I know - a user with a single login can have multiple blogs. But my impression (and believe me, I don't *know*, so am trying to confirm), is that it is the *login* which matters, and not the email address for keeping things straight.
The *reason* I care about this (fwiw), is I am converting a bunch of existing user sites from a proprietary system I have into mu sites (at least, that's the current plan). Each user has a unique username (login), and most users have a unique email address, but *not all* - an example is similar to what andrea_r mentioned above, where two distinct users are partners, each having a distinct login, but share a single email address for the business.
Hey, if the email addresses have to be unique, then so be it. I am just trying to figure out if this is true, and why.
Thanks for your patience on this.