There's a lot of really good point here. I hadn't really considered Martha as the ramp down companion, but in that sense I can see the purpose her crush served to the overall story (aside from making it clear that Rose was different).

In fact, every time that the Doctor took on someone else, it was made clear that the Doctor and Rose were a unit and other folk were nice but not necessary

Yeah, pretty much every time there was a third person on board it was the Doctor and Rose, and one more. Whether they were conspiring for her to show Adam the way of things or their dance around in front of Jack or their insolur world not including Mickey, it was *them* and their companion. And then Jack comes back in season three, and suddenly he's the Doctor's companion at least equal to Martha if not closer, it certainly wasn't two and Jack; if anything it was two and Martha (or three and Martha since Rose was so *there* in that story).

made very clear when he calls her over to help him with the wiring and takes her out of the 'active fighter' count

I just love how lady-programmer picks up onto that instantly. The Doctor plucks Rose out of their army because he will not let her go into danger like that and just like that it's clear that she isn't fighting. Nothing for Jack (companion) and Lynda (almost companion), just Rose because she's different from everyone else.