I'm considering that maybe people want the ease of simply logging in with their email, and the simplicity of entering (perhaps autocompleted from contacts) their friends email to set up a messaging connection with them.<p>I'm also considering that, maybe emails and spam are already so overloaded, people may value "a fifth channel" they can use out-of-band to chat with whom they choose. Say I can share my handle with a subset of people who may or may not intersect with those who have my email. Or I can publish my handle, while keeping my email secret.<p>If anyone cares to share some thoughts or experiences, that works. The Q's not asking for a prediction of the future, just a reflection or a tale from the past.
Offer both. As an example: the Threema messenger gives you an ID (cryptic, but could be a choosen handle as well) and allows you to add phone numbers and mail addresses as identifying information.