The 5d mkIII doesn’t have a built in one? (I’m not familiar with the Canon line, but that surprises me) I’ve always understood that it’s less about the full frame and more about the pro-level of the camera. The pro cameras are expected to have a vertical grip, they also need more juice to run for longer shoots (and to run some of the more advanced features—my D300 is limited to 6fps without the vertical grip (extra battery), if I have the extra battery in there, it jumps to 8fps)