I don't get this. Anytime I bring down an enemy close to death (even creatures like trolls and spriggans) they "yield" to me and just sit on one knee for a while. So say I decide to spare them, I shield my weapon, but then they just get up again and go right back to trying to kill me? What's up with that? Every single one tries to yield, and not a single one surrenders when I give them the chance... so then what's the point of them trying to yield in the first place? Is that some kind of bug or am I missing something?

dont bother sparing them. It's just a little thing the developers put in to make the combat more visceral.

They're not yielding.

When NPCs are close to dealth, they kinda flop over in pain and weakness. You just have to finish them off.

When they do this, dont sheith your sword at first; decapitate them then feel free to relax.

If you leave them alive then he'll pulll himself together, get up, and try to kill you once again.

They dont stop when you're about to die. FINISH THEM!

They aren't actually yielding? Then why do they say "Enough! You win" or "Victory is yours, I yield" and I think there's another line they same that is similar.
And decapitate you say? I just use a bow.

As tough as Skyrim is, there have been plenty of instances over the entire TES universe that show there are cowards among us.

I find it quite...unrealistic, that if they cower in deafeat, they don't accept it. Not a single one....Mayhaps in a future update, there will be a chance modifier. Depending on the NPC's level, it will flee or fight.