There are some set ups that will suffer from a slightly under powered shot more than others. For example setting up a Hill style longbow with an aluminum arrow that only flies perfect with an absolute full draw with a perfect release. Anything less and that aluminum arrow can wobble for 30 yards. Hill shows in videos that at times there were variances, Byron Ferguson talks of shooting a shorter longbow for hunting as a short draw insurance, I am not as good as either one of them so I figure that I at least need an arrow that can tolerate some inconsistencies in my power levels. I check to see how my arrow flies with a weak shot. A 475 grain arrow that can tolerate a short draw and flying directly behind a broadhead will out penetrate a sloppy flying 575 grain arrow most of the time and has a chance of being more accurate than a heavier arrow that is over spined because of a short draw. Going heavy is a fine thing, but it still needs to fly and match the shooters tendencies.