Why do people continually discount the advantage of a pistol grip in certain situations?
There is little to no advantage of having a pistol grip on a semi-auto rifle.
The purpose behind pistol grips on fully auto rifles
is about control, used to help counter the effects of muzzle rise during long round bursts (usually of higher caliber than the .223 or 5.56). The grip made it easier to stay on target while throwing hundreds of rounds a minute down range.
Mostly it's just for show on civilian guns in all honesty.