Of course, having worked in care for many years, I could also point out that there's never been a companion with a disability. None of the Doctor's regenerations have ever had Down's Syndrome (although it could be argued that the Doctor has Asperger's Syndrome). And that TARDIS, not exactly wheelchair friendly, is it?
Well as a guy with a disability and who uses a wheelchair, no matter what the uber-positives always say, there ARE certain things we can't do. We are not able to run with the Doctor and escape Daleks or Cybermen. There should NOT be a disabled companion because of this. Not a main one, anyway.
And while I'm sure the TARDIS can make itself accessible for the needs of its inhabitants, even I wouldn't want to see that.
But I think it would be nice to see people in wheelchairs on the show. People who work, or assist the Doctor at UNIT or in a lab. Or even show one of the companions with a boyfriend or girlfriend in a wheelchair. That would be nice to see. THAT is something under-respresented on TV and movies that I would like to see more of.