But that statement is idealistic, instead lets be realistic. Even the best of hunters and marksmen can have situations arise where the animal is hit and goes down but doesn't die quickly or even after a prolonged wait. And that can happen with most of us that aren't perfect. Then a finishing shot is human.
I once shot a nice fat whitetail doe in south-central Kansas, last deer I've shot or even went hunting. I was using my TC Gray Hawk 50 cal. and came on this doe with a yearling spike buck at close range, maybe 30/35 yards at most. Surprised us both! Just as I dropped to a knee to steady and almost simultaneously make my face-on shot she saw my movement and quickly lowered her head under a cedar bow to see me just as the ML fired. My 245 gr. Buffalo Ball-et hit her approx. 12 inches behind her skull and severed her spine. She dropped like flipping off a light switch. After a few seconds of absolutely no movement I walked up and she was breathing and eyes going wild even looking straight at me, no blood except for a small staining around the entrance hole. I reloaded my rifle as quickly as I could (slow) with a half load and instantly stopped her fear.
If I'd had my Lyman's Plains Pistol that torturous time for both of us while I gathered powder, patch, ball and cap could have flowed much much faster.
There are good reasons to carry a second firearm both while hunting and for self defense. What if that harmless doe was a mad boar hog and I missed the spine? A second firearm can save your hide!
But I understand and am sympathetic with your argument statement.