I disagree. In the vast majority of situations, monsters are looking to accomplish a specific objective that isn't "kill or incapacitate all the player characters." What I outlined above was that, in many of those situations, killing unconscious PCs could advance that objective more effectively than another action.
Maybe it varies from table to table. In my experience, the primary goal of any monster is to survive, and that's not something which will happen if they leave any PC alive. They're
monsters, and heroes
kill monsters.
There's some more wiggle room when it comes to intelligent NPCs, or minions of the big bad. There
might be a case where killing a single PC will cause the rest of the PCs to back off. Sometimes. Usually, it will just make them mad.
Even then, though, the concept of lasting injury is somewhat foreign to the world of D&D. This isn't L5R. You can't seriously injure anyone without bringing them within a hair's breadth of death. And combat rarely lasts more than a few rounds, so if you're giving up one of those rounds to finish someone off, then you're sacrificing a
lot of your combat potential; if you couldn't drop the whole party
without skipping your turn, then you have significantly less chance of doing so
after you waste a round.
At a meta-game level, if the DM is expected to tailor encounters to be tough (yet beatable) challenges for the group, then an enemy who wastes a turn on such a maneuver will shift the whole encounter from
tough to
easy. In order to have any chance of winning, they really need to go all out on offense rather than playing mind games with a coup de grace. That will vary between tables, though, since not every table uses tailored encounters.
The real exception is if you can perform a one-hit KO, in which case you
might convince the PCs that you're more trouble than you're worth. If you can't clearly demonstrate that you're way out of their league, though, then you've just signed your own death certificate.