Stem cells replacing cancerous cells?

Not really because cancer has their own stem cells (check out the cancer stem cell hypothesis). So if you leave behind one of the CSCs it has the possibility to regenerate the cancer. Some times it is not feasible to remove the cancerous region (hard to reach portion of the brain, body-wide metastasis, etc.). Stem cells cannot really outfight cancer cells as stem cells are usually dormant and slow growing where as cancer cells are more proliferative. Good thought but it gets a lot more complicated.