I've probably deposed 50 psychiatrists/psychologists in my day, and consulted with probably an equal number. Doesn't make me a psychologist, but it does mean I know what a defensible diagnosis requires. And that's including diagnoses with the DSM-III, III-R, IV, and V.
There is a huge modern tendency to describe personality traits as disorders -- essentially lowering the standards for what actually constitutes a disorder. Mostly, it's done by lay people, or non-psychiatrists/psychologists, though pop psychiatrists/psychologists will do the same thing on occasion. Anyway, every single one of the 100 or so I've deposed/consulted with has said that a personal interview is mandatory if you want to have a reliable, defensible diagnosis. There are simply too many variables that cannot be determined otherwise.