Yeh, I mean you can do without it. FusionPBX lets you log in with an email address, it's just that FusionPBX uses it to separate tenants in the authentication and provisioning process so it's kind of essential if you're using the multi-tenanting. Otherwise you end up with iffy stuff like the same provisioning credentials across domains, which isn't a *massive* problem, but could be a problem if the directory link is leaked, or maybe some crazy stuff with MAC addresses...but 3CX seems to do alright just using the MAC address as security so who knows.
Oh yeh, and if you don't use separate domains partners have to rely on using the crappy format username above instead of their email; but that's primarily because the domain selector is an all or nothing kind of thing, so you can see all of the tenants or none.