The user login (hopefully) is not the internal primary key for the user. It should be unique at a given point in time, obviously, but certainly there are reasons it might need to change.

And I've been bitten by this design numerous times. The most common bug: literally the birthday paradox, i.e. "these two people have been assigned the same number, and now we need to distinguish them in our database".

All of them do I think. But from experience, they can change it for your account.