When someone bails from their aircraft in a dogfight, they drop below it, obviously. So to get guns on a chute, you have to burn more E to do so, making you vulnerable to other attackers.
So really, if you shoot a chute, you're showing just how good a pilot you are -- that you can afford to take this risk, and you're telling the guy under canopy just how much you care.
Plus, if that chute guy takes it the wrong way, only good things can happen. He may grab the nearest Spit and charge back to challenge you again, allowing you to repeat the demonstration. Or he might just ***** about in on channel 1, which does you a better service than the kill buffer ever did.
Gentlemen shoot chutes.