Scenario: One of your users forgets his login information, so that person tries to re-register. This causes duplicated accounts and a source of frustration for you and your members.
Solution: When a user tries to register another account with this add-on installed, they are presented with a friendly error message that an account already exists with that email address. They are presented with a link to the reset-password form, already pre-filled with their existing username.
The core has a duplicate username check already in place, I just modified that code and attached to the delegate right after that point.
While I do think it's essential, I can think of a couple scenarios where it wouldn't be best added to the core: A support team that uses Vanilla for product support all shares the same email box, and a online game that allows role-players multiple accounts. Plus, I can't help but realize that Vanilla got along fine for more than two years without this, and only one user requested it (AFAIK).
i would think that a vanishingly small percentage of vanilla installs would NOT require duplicate email checking. i have always assumed that it was in the core.
"option that's off by default" sounds reasonable to me.
this addon should should also add the functionality to reset your password by email address. (if they forget their username as well as their password.) like enter email or username to reset your password.