Reuse of IUserIdGenerator?

[jensens]

"Keep in mind that plone.app.users already makes use of IUserIdGenerator and ILoginNameGenerator utilities (https://github.com/plone/plone.app.users/blob/master/plone/app/users/browser/interfaces.py#L20) so please consider whether the ones you're adding here duplicate that functionality." (by @davisagli in #12)

Some research needed if this makes sense in some way, at a first look this has some side effects.