In the end, I think the real problem with Martha is that they only had a six-episode story to tell with her (Smith & Jones through Gridlock and Utopia through Last of the Time Lords). She would have worked better if she hadn't stayed the whole season.

Oh, oh, brilliant - there we go. Just poor planning on TPTBs' parts I guess; I don't think they realized how tedious her arc would be and how unnecessary it was to drag it out and emphasize it the whole season. As you said, she had a specific purpose to serve, and she had her own arc, but there wasn't enough investment in her as a character to sustain her presence the whole time (nor was Freema able to add to the writing). At least we should see something completely different with her in the future though, and she should be much more palatable in small doses, with tight arcs. I'd be surprised if she doesn't get more development in three eps of Torchwood than she had all season in Who *sporfle*