This is the season that is covered in The Writers' Tale and I'm trying to recall what is said about Martha there. Nothing is coming to mind though. I don't think there was particular concern that Tate couldn't carry the companion roll. Mostly what I remember is Davies endlessly worrying about how to resolve the Rose plot line.
IIRC there were vague plans afoot to move Martha over into Torchwood, so its also possible that her presence here was somehow intended to segue into that (though her exit at the end of Journey's End) makes more sense of that. Of course, she may be supposed to serve a similar purpose to Adam in season 1 and Mickey in season 2, to shake up the dynamics a little and highlight the companion via a contrast, but then shunting her into a separate plot line makes no real sense.
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IIRC there were vague plans afoot to move Martha over into Torchwood, so its also possible that her presence here was somehow intended to segue into that (though her exit at the end of Journey's End) makes more sense of that. Of course, she may be supposed to serve a similar purpose to Adam in season 1 and Mickey in season 2, to shake up the dynamics a little and highlight the companion via a contrast, but then shunting her into a separate plot line makes no real sense.