I think that the line that should be drawn for cloning is when a being is sentient, and I think that is a question that Ishiguro is so cleverly getting across in the text - how can a society allow sentient beings to be bred for their body parts?
Were Madame and Miss Emily just championing fairer treatment for the cloned generations or were they pushing for it's end when they tried to prove the souls in these clones?
I would be happy to receive a body part grown in a test tube if I so needed, but a part grown for me in my cloned self? I hope that I would value the soul of any clone over the physical need for a cure. A absolute biological match would be the dream donation, but not from a being.
As far as availability of cloned donations, as with many of life's realities, I think the technology would favour the rich first, before an eventual filtering to the poorer. We have that now, where the poor in many countries sell their kidneys to a person willing to pay.
There are lines across which we as a society should not step, but I believe it is a line we will reach before we have decided where we want the line to actually be.