1) legacy support
2) did you try it to see what might happen?
>2) did you try it to see what might happen?
I created a user with Inline Password and specified a User Login Domain for him; left Windows Domain empty.
As result, I can log the user in with either specifying the User Login Domain or without it - does not matter. So, why is the User Login Domain for in such case?
Inline password stores the password in the database the content server autheticates against this value. It does not try to authenticate against the domain; therefore, as you stated, defining user domain is optional, since its not used.
EMC left the attribute there for UI consistency. If you look further, this attribute is NOT required like many other user attributes.