Right. As a routine action, in D&D, you should exercise common sense in your fantasy realism.
That is, unless there's someone bearing down on/stopping them, any intelligent and/or trained enemy will want to make sure when you go down, you stay down. Particularly in the case of inherently evil (e.g., orcs or hobgoblins), savage (e.g., ogres or gnolls), or successful meek (e.g., kobolds or goblins) creatures, they'll do it just for the sadism/gore/cruelty/asserting their dominance/expressing their hatred for the race of the creature that went down...any number of "wicked/evil/perverse" reasons they will give you that one more stab or twist of the blade after you go down.
