Well I do think there is a bit of a hole in your logic. A mastectomy kills tissue by removing it, and you are right, you are not going to regrow a breast.. Radiation kills cancer cells by disrupting their DNA. The skin graphs are needed for reconstruction, whether or not you have radiation. You are planning to have reconstruction because you had your breasts removed, not because you did or did not have radiation.
As somebody who had a bilateral mastectomy, medically necessary skin and tissue transplant, chemotherapy, and radiation, I'm here to tell you the easiest and least invasive of all of that was radiation. And all of it....the surgery, the chemo, and the radiation....are just a guess that it might help and it usually does. So why opt out of the easiest and least invasive of all of that help?