Just that really.
I've always logged in with my username but now we have to log in with our email address.
People's email addresses are hardly kept private - lots of people know mine, including multiple family and friends - and of course, any bank/service provider/offer/competition/doctor/dentist and so on.
I tend to use the same password, or a close variation of it for many things. And i'm pretty sure that if, for instance, my dh or one of my sisters or my bestfriend wanted to try and bust into my MN account (not that I actually think that would happen!) then they'd have a much easier time of it using my email address than trying to guess some random username that I've made up.
AIBU?