This argument cannot be broken.  I have often heard people say that a well socialized dog can still guard.  While this is true it will not excel as a guard dog if it is overly socialized.  To be socially accepting of strangers (socialized) is contrary to the suspicious nature really needed in a real guardian. 

Depending on societal needs of your location and the litigious and political climate - it is proper that some people do not bring a true guardian into that situation but find a more mondane breed.   Good discussion everyone.