Great Question!  We thought the same thing when our oncologist never talked about when Lori would need to get a mastectomy!  Turns out, for breast cancer patients with distant metastasis (to a distant organ or bones), there is no cure and there is no data that shows survival rates are increased by performing a mastectomy.  A mastectomy also introduces unnecessary risk in this case.  The emotional and physical pain can be severe, there is risk of infection, and the patient must forego cancer treatment (chemo, trial drug, radiation, etc) until the wounds heal from the surgery.  This delay in treatment provides a better chance of hastening death than simply letting the tumor exist in the breast.  Furthermore, the tumor in the breast serves as an excellent source of feedback for how the various lines of treatment are working against the cancer.  If the tumor shrinks, the treatment is working.  If it grows, the treatment is not working.  Instant feedback!  Finally, a breast tumor is a very easy place to perform a biopsy, and biopsies are performed relatively frequently for things like genomic mapping, trial drug applications, and other necessary readings.  In a way, for patients with metastatic breast cancer, that tumor is your best friend.