LoginName is used for authentication, as such it is private data that I don't think should be passed in an url. Also I would not want to make the userprofile page respond to that in the url because that means people can try manipulating the query string and if it works they have discovered private data about the user that should not be discoverable, ie leaking private information.
The integer id or guid does not leak any private information and has no intrinsic value or meaning.
Best,
Joe