There is a growing trend to allow users to login using their email address. PaperCut version 14.1, allows users to use their email address as an alternative to login by username to it’s interfaces. (i.e. user, release station, MFDs, web release, mobile client)
In some organizations users don’t even know their username (sAMAccountName); this would allow them to login using their email address.
It is also useful in cases where a user has forgotten their username.