I have a few thoughts about logins. Especially on a phone, but even on a desktop, it is an inconvenience to type out email addresses, due to symbols and the cruft of the domain name and tld.
However, email addresses are convenient because they are automatically unique. Being unique already is a nice bonus for a website needing a unique identifier for accounts. And the fact that most services need emails anyway, for contact, and human verification, I can see why emails are a necessity for most services.
Usernames can add a nice personal touch to a website. People can use the real name or a variant or any reaction thing they want.
Usernames are usually shorter than email addresses, and always contain less symbols (my bold statement for this). Typing out a username has always seemed slightly more pleasing than typing out an email address.
No evidence except personal experience for this, but I seem to remember my usernames per site much easier than my email addresses power site.
My conclusion is that both email addresses and usernames have value and I think that any decent service should allow login with either of them. I decided to allow both while developing Daypage.