I guess I will try initially to just stick with the book, and show the danger of disregarding a fallen ally by making an enemy finish them on the ground. I will not use that in any fight, but will wait until they get to one against smart foes, who can plausibly respond to their preferred "whack-a-mole healing" tactic. Hopefully, it won't take more than one casualty.
Well, you know the psychology of your players and we don't...
I might say, though, that even with the somewhat weak healing of 5e, it is not always a losing proposition to heal. There are some good cases when healing does come ahead.
Absolutely true - unfortunately, the clearest case is when you heal a dropped ally just enough to take the next hit without risk of being instantly killed - that is, when leveraging heal-zero and dying rules.
This might happen when the enemy has better defensive capabilities that reduce overall damage output from the party, making offense weaker than what the numbers would suggest in the first place, combined with the adventuring group itself also having good defenses.
Nod. Those examples also make for longer combats, which have different dynamics in a variety of ways - including healing being a relatively 'stronger' option by the numbers (though, it's also less likely for allies to be dropped in the first place, so healing may also be less critical at any given moment).
Which just reinforces the point that different scenarios have different optimal choices.
The trick can be getting players who are in a tactical rut to /see/ that, too, though.
But trying to get that desperate last hit to stop the damage from coming is kinda guessing game too. And one that they are most often quite eager to try, even on some situations that they already know the odds are heavily against them.
Yep, maybe it's that it's one you see the result of right away (the monster drops or it doesn't) or maybe it's that the damage you do is still contributing to dropping the monster?
Not only that, but if that (let's say fire) giant hits the newly re-awakened hero, the bulk of the damage dealt would be wasted. Look at all the "damage" not taken by the party because someone only took 6-8 points, instead of 28! That's an effective savings on healing later. If the downed PC had not been restored (for those 6-8 HPs), that fire giant might have hit someone else for full effect. Potentially draining even more resources after the fight.
That's part of the argument for standing up a fallen ally, even with a fairly weak heal, yes, but the example was of prophylactic healing - the rogue was still up, but didn't have enough hps to take a hit, the hope was the 6-8 would be enough that that he'd still be standing after a hit from the giant. Which makes some sense to try if the initiative order goes healer-giant-rogue, rather than rogue-giant-healer... but that's getting into criticisms of cyclical initiative.