I think a better question is why not shoot a heavier arrow of 10-12gr/pound of pull? I mean, there are pluses all around. Quieter, better penetration, more FOC usually. I am not sure why people would want to shoot a 400gr arrow?
Answer would be to help cut back onthe high low misses that come from making minor miscalculations in distance (consciously or instinctive) that happen when shooting unmarked yardages. Real easy in the woods to "see" a 22yd shot as 17 or even 27 - especially when stalking or still hunting and shots happen quickly.
Deer are hardly ever lost from poor penetration on an accurately placed shot.
Most unrecovered animals come from a poorly placed shot.
My bows are quiet at 400 to 420 and have no problems with penetration on shots to the vitals.
Another 100 to 200 grn would not help in a gut shot or other poorly placed shot.
My question would be - why would I go heavier - make pinpoint yardage more critical - and likely up my odds greatly of making other then good shots on game?
Steve