I knew you would like it ;) Click on the link I provided, some great and interesting answers there. Basically, Doctor and Baby are used as substitutes for names and therefore I shall spell them with big letters. And here I was trying not to, as not to be confused with The Doctor ;)

Apparently people like teachers also use it with just referring to "Mom" when talking to a child, or the kids just say "Teacher". It is a bit odd though. In "Glee" they say "Mr. Shu" as an endearment for "Mr. Shuster".

As for "the wife" most say that it's usually said in joke?