This io9.com article was the proverbial straw: We Asked Russell T. Davies Our Most Pressing Doctor Who Question.
That question: “Why exactly is the blonde ingenue Rose Tyler the most special of all the Doctor's companions?”
I don't think she has been [treated as special].* I don't think I feel any more special when I'm writing Rose than when I'm writing any of the others. I think there's an iconography about Billie Piper. When the programme came back, it was the biggest advertised, most hyped-up programme in the world [and she was at the center of the imagery]. I'd never prefer her to Donna or Martha when writing her. But she was enormously popular and so — let's be blunt — every time I brought her back, the ratings went up. It's my job to make people come back to watch this. Sometimes people roll their eyes and go, "Oh, you've got another returning character." [To which I respond] "Yeah, leave me alone with my millions, thank you very much." So you know, it simply works. Plus we like Billie... So simply by dint of being her, she's come back the most often.
Not treated as special? Really? Even though she gets an AU replacement for her dead father and her very own Doctor!clone? As for those rating boosts, a commentator points out that Rose’s reappearances occurred during season finales, when one would expect viewing figures to go up.
Genuine cluelessness, or an elaborate leg-pull? I don’t know, and frankly I can no longer afford to care.
* Headline: Optometrists Report Eye-Roll-Induced Strabismus Epidemic among Doctor Who Fans
ETA: Y’know, RTD may single-handedly convert me to the Death of the Author theory.