If I am carrying a high powered rifle, I don't need or want a sidearm, as it isn't anywhere near effective at stopping a charging bear. I have been charged by a grizzly, and I did use a high powered rifle to end the charge, so I speak from actual experience. Now if I was fishing or bow hunting, I would like to be able to care a large bore handgun. In any case, I would not be carrying a 9mm or 45acp handgun for protection against animals.
When a bear sticks his head in your tent, that's when the handgun is beneficial. When you leave the rifle leaning against the side of a tree while you're skinning and butchering, or just addressing the call of nature, that's when the handgun is beneficial. Its not about which gun is the better stopper, its about what you wear as opposed to what you carry, and its about what you have with you, as opposed to what you have to go and get.
The problem a hunter has in a defensive wildlife scenario is that he's programed to take a chest shot no matter the circumstances, and if he sees a relatively small animal run off after being shot in the chest with a rifle, he isn't inclined to think very highly of a gun that's harder to use well, is less intuitive, and has a third of the velocity of his rifle. The thing is there's more to it than that.
Despite it's low velocity, a handgun with suitable loads penetrates exceptionally well, and is usually enough to exit on a head shot, turning everything inside the brain pan into a jellied mess. So if one does choose to carry a handgun defensively, he must load it with the right ammo, he must shoot it frequently to ensure he can score a decisive blow at 20', and have the knowledge of what a decisive shot on wildlife actually is. Compared to your bare hands, the handgun is decidedly a step up.
The bigger a handgun cartridge is, the more difficult it is to use, particularly if multiple shots are required. A 9mm with 147 gr bullets approaches the terminal performance of a .357 magnum, and it carries a bunch more ammo in the magazine than the .357 has in the cylinder. My carry gun is a .44 magnum which I load with 325 gr bullets over 20 grs of powder, the resulting 1200 fps produces 30 ft-lbs of recoil, and pushes the gun well off target. A .300 magnum produces about 35 ft-lbs of recoil, and if shooting off hand that amount of recoil complicates a fast followup shot, unless you practice incessantly, never mind taking a similar amount of fast recoil in the palm of your hand. A talented pistol shooter with a 9mm could probably fire 5 deliberate rounds by the time I fired a second one with the .44. If the the 9mm bullet punches through the animal's brain pan, what does the .44 do that the 9mm doesn't? My only 9mm has a 3.5" barrel, and its a difficult little sucker to shoot very well due to the small, hard to see front sight, but if I had a 5" model with sights I could see, I wouldn't hesitate to have it added to my ATC, and park my .357 which currently backs up the .44.