Now that I think about it some more, there's different arguments for/against coming from different premises. Someone could argue something like having to finish off that crawling enemy creates an interesting potentially emotional decision moment. Someone else might just think hey that's cool for the sick/perverse factor , or feeling like a real bad guy, or the opposite as you say not wanting to see them your enemies in agony. Someone else could make a simple practical argument against prolonging death because they just wanna loot that dead body asap and move on.
Going back to my comment about it not always meaning instant death, I guess at the very least that would provide some variety. Variety is nice, not seeing the same outcomes every time, so yeah I still say let's have instant death sometimes but not always. That variety also spreads things out so everyone from those different arguments is at least pleased some of the time.