or maybe its because everyone will acceept someone as beeing "a doctor" no questions asked..
for example
old woman> who is it dear?
old man> its the doctor
end of.
old woman > who is it dear
old man > its barry
old woman > barry who?
old man> shut up you old bag its just barry stop asking so many bloody questions and put me tea on.
why over complicate things with stupid meaningful crap like moffat