Lets consider the scenario of a 200 yard broadside shot on a deer. There's no wind, its a bright cloudless morning and you're shooting from a comfortable solid position. You hold a third of the way up the body just behind the shoulder. But as the trigger breaks the deer takes a single 18" step forward. Thats a gut shot! It doesn't matter what kind of rifle you're using, it doesn't matter what the quality of your scope is, if your markmanship is perfect but your target moves 18", you've got a wounded deer.
Now lets consider the same scenario with the deer shot on the length from behind. You're aiming for the spine, the deer takes an 18" step, guess what, the deer still has a severed spine, and can't run to the next area code. Will you need a follow up shot? Maybe, it depends on how much momentum your bullet has at 200 yards, but I've seen a spine shot deer hit with a 180 gr .308 drop like a rock. I still remember cutting my finger on the sharp bone when I went sticking it where it didn't belong.
So why is it that a broadside shot is considered by some to be ethically superior to one made on the length? Maybe its because if a hunter carries a mouse gun, broadside is the only option he's got.