I never used to carry extra. All I carried was a 5 shot 44 magnum & no extra ammo.Then I got it a heated debate about that very subject.It went something like this. My thoughts were 5hollow point300 grain bullets should be enough to bring anyone down. Now here is the answer. How good a shot are you? Do you feel that you'll never have to repell an attacker from more than 8 or 10 feet? Do you want to be ready for anything ? My answers were that yes that's the reason I carried a large caliber handgun with a little longer barrel so that I had a better chance of hitting what I was aiming at & enough power to take down an attacker.
1. The only shot that will stop someone 99.9% of the time is a shot to the central nervous system (brain or spine). Any hunter will tell you you can almost never drop an animal in it's tracks. But animals don't shoot back.
2.If someone is shooting at you I don't think you are going to stand in a weaver stance & shoot back. no you'll be running for cover & trying to defend yourself at the same time. If you have a place to try it set up a BIG target. Run & shoot at the same time. My guess is you won't hit squat.
3. Adreniline is going to be pumping so that you'll empty your gur in about 5 seconds or you'll not shoot at all. I vote for poion 1. That being said even if you hit your attacker 3 or 4 times (which I doubt) they could still be a threat. Not to even get into the fact that your attacker could have a rifle or shotgun.
But then again it's not always practical to carry a big gun & lots of ammo. A 25 in the pocket is better than a 45 at hme.
Youcan NEVER be completely prepared for anything. so I guess it just depends on how prepared do you want to me
Just my 2 cents