butterfly | Doctor Who: Companion-related thoughts on 'Sontaran Stratagem'

So, RTD apparently says something along the lines of "Martha is the Doctor" in the confidential for 4x04 (I have not watched said confidential yet, but if I watch it and he ends up actually saying something like "Martha is a doctor now," my conclusions remain the same, because while this post was inspired by said comment, it is not dependent on it).

I would agree with him and would, in fact, expand it outward.

If Martha has grown to emulate/mirror the Doctor over their time apart, she's far from the only one. In fact, it's quite the popular thing among former companions. Sarah Jane is merrily performing a very Doctor-eske role over on SJA, Jack remade TW3 over in the Doctor's honor, Rose and Mickey are 'Defenders of the Earth', and Donna was directly (and hilariously) mirrored with the Doctor in the opening of PIC.

The Doctor exposes people to a new way of living your life (as Rose said). He shows people new skills to deal with the problems they face. He inspires people. Companions are all people capable of rising to the challenge of the Doctor's life and, while living that life, they learn things. They take that knowledge back into their lives without him.

Not only do they all learn from their time with him, they all learn slightly different things -- all of these former companions are 'defending the Earth' but they're doing it in different ways. Jack first worked with and then reworked Torchwood. Martha seeks to change UNIT from within the organization. Sarah Jane avoids both of those organizations as too prone to rash action and prefers to work without any official affiliation. All different responses to what they learned from traveling with the Doctor.

"Someone has to be the Doctor," Rose says in The Christmas Invasion. When the Doctor is missing or out of commission or otherwise occupied, other people have to fill in that hole. Former companions tend to be the most qualified for the job.