This Year's Model of Humanity

Martha's character arc may have theoretically been intended to serve a long-term narrative purpose, but as executed it fell totally flat on its face. I think the fault is mostly that of TPTB. The writers got very lazy with the writing of their main characters in Series 3. Whether this is because they were used to having two leads who just completely inhabited their characters, I don't know. But as I've noted elsewhere, you can't severely underwrite a role unless the actor's capable of essentially writing it for themselves in their head (e.g., David Tennant, Billie Piper), or is largely playing him or herself (e.g., John Barrowman). If they wanted to allow the writers to sketch their characters in shorthand, then TPTB needed to cast someone with more experience/confidence. Knowing that they had cast a newbie, they should've leaned on the writers to be very specific and very thorough about what they wanted out of Martha.