Eh what situation are you referring to mate? here is the deal why should i use expensive pg's when i get more flexibility and firepower with the flmethrower pio's? in fact some higher level players you see players going straight for the double flame pio's and rarely bother with pg's.
The one shown in Australianmagic video.
PG are not meant to be get 100% of the time nor are non brainers as getting LMG grens. I feel like it's harder to justify more than one of them on 1v1 but the thing is that you don't always have enough munitions to get LMG + flamer + medics + tellers around, so PG is a MP option to have appreciable DPS.