That's a good point too, Halo. What's been explained to me in different discussions, is that the .50 cal's and other weapons are feared because of their range. A .50 cal could accurately take out a VIP target at up to a mile or more. That is outside of the protective cordon that people like the Secret Service or police could cover to protect someone like the President, Sec. of state, congressman, governors, celebrity's, etc. Plus, By simply being farther away, the assasin would have more lead time on potential pursuers. Add to that, a large cal. weapon like that 20mm. firing an HE round would guarantee a one shot kill, and likely one or more "collateral" deaths. It is, after all, what those things were made for.
As for sporting purposes, African bull elephants' used to be taken with something like a .458 Win. Mag, or .416 rigby, .505 Gibbs, etc. None of those cartridges are quite as powerful as even a .50 BMG round. However, I would say that a .50 BMG would'nt be out of place on an African dangerous game hunt.