This is assuming the knife guy is trained. We used to play around with this in the academy. If the pistol holder is trained in using his pistol well the knife guy has two in him before getting into range of said pistol guy.I agree someone HIGHLY trained with knives can pose a serious threat. But someone equally trained with a pistol can be VERY fast and accurate. It pays to train.
This is a very good case for open carry, it's just faster