why do we (self included) spend the extra money on high performance ammo?
Getting back to this original question, there is, to me, an obvious answer. And that would be faith. Allow me to explain...
We know that expanding bullets have a track record of being more effective in many situations than non-expanding ones. And we are putting our faith in this. We know that when the bullet goes in exactly the right place, it works, no matter what kind of bullet it it. But the bullet getting in precisely the right place is dependent on many factors.
The biggest single factor is the shooter's aim. Then what the bullet has to go through to get to that "off switch". That is where the size, shape, construction and speed play their part. I can discuss this in detal, and will if that's what you want to do, but for brevity's sake I'll ..condense things.
We put our faith in the premium ammo, expecting and believing that it's superior performance, (which by some standards is only a marginal improvement over the lower performing ammo - after all, handguns are all "weak" when compared with something more powerful) will be the difference between sucess and failure when the bullet does not get to precisely the right place.
The difference between a bullet that wounds (even severly) and one that stops an assailaint may be fractions of an inch in the precise path taken. IT may also be there is a certain energy threshold needed, so the faster the bullet the better the odds, as well.
Look at it this way, on the range, calm, unaffected by the extreme stress of a life & death situation, how well can we put the bullets exactly where we want them to go? Pretty good, yeah? but never quite good enough, or not always everytime, so we practice. In the much, much different situation of a defensive shooting, with all the stress, and the difficulty of both target movement and precise target identification (point of aim) anything that we believe will increase our odds of success, no matter how small, is worth using.
What is the possible down side of using high performance ammo? Assuming it works at least as well an lesser performance ammo in terms of accuracy and functioning in your gun, then the only possible downside is the cost. And the old saw is "what is your life worth?"
Now, if the "high performance" ammo won't work in your gun (won't feed reliably, won't shoot to point of aim - no matter what you do-, won't group well enough to be on target, is too hot for your gun to handle, etc.) THEN you have a different issue to consider, and perhaps a different conclusion to the question of "is it worth it?"
How much, of what is needed? That comes into play in the decision process, as well. And what does your gun & ammo combination actually DO?
It is something you need to test, for yourself, before you settle on a decision. I once had a gun that shot excellent groups, exactly at point of aim at the "usual" gunfight distances (short & medium) with a standard .38SPL load. Same gun with .357 ammo was nearly a foot low & right, and groups were just ..fair. Now, with this gun, what would be the best choice for defensive ammo? The .38 load that goes right where I put it, although considered weak, and not the most effefctive? Or the good magnum stuff that has the best stopping power rating, but shoots like crap in my gun?
Meaning I can't be as certain that the bullet will go where I aim it? Kentucky windage is a useful skill, but its tough to remember to use it when your life is on the line, and at speed.
Don't take it on faith that because it costs more that it will work better in your gun. Test it. It may be completely superior in everything bullets do, in the target, but before any of that counts, the bullet has to hit the target, and hit in the place you want and need it to.
Once tested, and confidence obtained that the high performance ammo will work fine in your gun, then it becomes a question of the cost of your training & practice. Ideally, the ammo you practice with should be identical to the ammo you will use in a defensive shooting, should one ever happen to you. But, that is expensive, particualry if you only shoot factory ammo.
What most people do is find a load, less expensive than the "high performance" ammo, that shoots to the same point of aim, and roughly duplicates the recoil level as the "good" stuff. This can be tricksy if you only shoot factory ammo, especially if your "good stuff" is a special high performance load from one of the specialty ammo makers.
You can, with handloading, duplicate any "high performance" load with a cheaper bullet for practice (assumming, of course, that the cheaper bullet will shoot about as well from your gun). If all you need is to puch holes in paper, a solid at the speed of the premium hollowpoint is a good practice round. Save the spendy stuff for "duty" use.
Now, maybe the cheapest GI grade ball ammo will be close enough in performance to be an acceptible training substitute for the expensive premium stuff in your gun. IF so, great for you! But maybe the ball shoots to a little different place, or maybe its a bit lighter recoiling, or for some other reason feels different enough to not be a close match to the premium stuff. So, something else is needed, if it is important to you to train & practice as "real" as practical. Note that not all our shooting is training for defense, most of mine is recreational. Plinking and such is fun, too.