Maybe it is... hmm... defining the Doctor by showing us someone that he isn't? If he really was just a normal guy (like bits of fandom would like to believe) then we (and Martha) wouldn't love him? So Smith does stuff the Doctor would never do like be all racist and hit children and go "YAY VIOLENCE" and throw Martha out? Though how does Joan fit that, as she is someone he would so shag if he was himself?