I'm going to be bad and not read all the other comments to see if someone has already said this. :)

Grace and Martha didn't both have to be doctors. The point, to me, is that they clearly had productive lives of their own, and plans that were important to them personally and worthy in general, that did not include madcap wanderings through time and space. It takes a lot of dedication to become a doctor, and they would have thrown away a lot of work to follow the Doctor.

I felt this way about Madame de Pompadour as well. She might have needed the Doctor to save her from being killed, but she didn't need him to have her full and fantastic life. The Doctor changed Rose. She became a bigger and better person for knowing him. Martha was already in the life-saving business.