LangLangC has given a very detailed answer as to how to disable the discrete Radeon graphics card in a 2011 MacBook Pro. It seems to be highly regarded, because it got a +97 score:
But there is also a Mac app called gfxCardStatus, which allows one to "Manually switch to Integrated Only or Discrete Only mode to force one GPU on or the other":
Does LangLangC's method have any advantages over simply using gfxCardStatus?