That's his point sandman. It's not the chute six call that's unrealistic, it's the whole system, where we can track and id several aircraft from miles away, whether in a tank, chute or aircraft. If you read accounts of WWII air combat, six calls weren't that effective: part radio discipline, part when the bullets started flying it was hard to tell which friendly was which, and part the radios weren't that good.
So it's a concession to gameplay -- you can hate it or like it, but singling out comms from parachutes as particularly unrealistic is untenable.
Hazed, pilots in chutes aren't defenseless. They can use comms, and they can (and usually will) replane after they land/die. Shooting the chute denies them comms, speeds their replane, and irritates them. Ok, if you want to make a good impression on someone, you don't do it. But if you want to make that guy get so mad he'll spawn from a vulched field and give you some more kills, I say it's the best way.
