Based on many of the responses, I guess today is that day
Here's why I disagree that doing HP damage is always the best option. There are two examples that immediately jump to my mind, although I'm sure there are more.
1. The math doesn't always support that unless you recharge all resources after every battle. If you've got a PC with 15 HP and does 8 damage each round, and an opponent with 15 hp and does 5 damage each round, assuming PC wins initiative, the PC takes 5 damage by the time the opponent dies in the 2nd round before it gets to act. However, if instead the PC has defense that reduces incoming damage by 3 and their own damage by 3 each round, the combat takes an extra round, but the PC only takes 4 damage by the end of the combat. So why offense is often better, it's not always better. The issue here is allowing PCs to recover all of their HP after every battle. There is no carry-over risk.
2. If you have 4 enemies each doing 10 damage each round, immobilizing one or two of them reduces their damage to 0 each round, allowing PCs to focus fire on those not immobilized and keep their own HP from being reduced as much. Clearly a better option that isn't just damage.
I do agree with you, but these options dont always exist / do not exist for everyone.
This was 100% the idea behind controller and defender rome in D&D 4e, but it probided specific mechanics for it.
Another problem is that you normally lack the information needed for this. Like you do not know exactly how many more attacks the enemy gets when you play defensively or how much less damage you take. But you do know that a dead enemy cant do damage, always.
You also dont know if not suddenly thst enemy has some huge trick like a spell slot etc. So killing it before it can use it is the default with a lack of information. (Especially in 5e where some really low hp enemies can have 4th level spell slots!)
Also you might fear a crit (especially with the hight crit damage rule which some groups use), so the longer an enemy lives the higher the chance they get some crit.
One other problem is also that often it can feel bad if I use a ressource and an action just to make the enemy also lose its action (snd we both deal no damage). Sure if its a single boss it will be worth it, but with more enemies it becomes harder. Thats why I think its good to always have also some damage on the control spells to feel some progress.


