Agree with the OP. Healing in combat becomes more relevant the longer the combat goes on, regardless of the number of enemies remaining. Healing is a resource converter. You are changing spell slots/actions into hp. PCs lose hp round by round and by virtue of attrition cannot fight forever. Healing extends the fighting duration and makes sense situational when it provides a greater benefit than using another action to shorten the fighting duration. When you get to round seven and all of the PCs are at single digits, spot healing becomes mightily attractive.
My reasoning on when to use in-combat healing:
1. An ally has gone down.
2. An ally will go down in one hit otherwise.
3. As a hedge against a critical hit.
4. To mitigate environmental, zone or chip damage to keep an ally out of the "danger zone". This is most important when you combine low-level ongoing damage with dangerous heavy-hitting opponents.
5. I am not in a position (or don't want to endanger yourself) to make an effective attack.
6. I want to save other resources, such as higher-level spell slots.
7. To help another player get a cool moment of spotlight time more safely. If the fighter has a cool idea for an action surge, but only 15hp left, give them a heal and let them shine a bit.
I, for one, am happy that in-combat healing isn't the superior option in all cases. It's great that it exists as a situational option that requires a bit of intuition and calculation on the player to manage effectively. In combat, healing is rarely wasted while delaying too long for optimal efficiency can result in disaster. A classic risk vs. reward/press your luck mechanic.
For DMs wanting to press the party into more combat healing territory, longer and more arduous combats are necessary. That requires:
1. Generally more opponents than fewer. Spread out rather than clumped to avoid easy AoEs or arriving in waves. If using a solo, you really need to go hard on the other points to make it a fair fight.
2. At least one source of consistent damage, environmental hazards, creature auras, AoEs, traps, spell zones, or multiple ranged attacks. Ongoing attrition is key.
3. Ways to mitigate the most potent PC tactics of focus-firing, save or suck zones and so on, to prevent alpha strike/nova-ing and shortening the combat significantly. Darkness, cover, anti-magic zones or whatever weirdness. Obstacles that can be circumvented with tactics are better here than blanket bans as you're looking to delay victory not make it impossible.
