I think "usually" would depend upon your group though as you say.
Let's say for the sake of argument that the bad guys are not hitting frequently. It's not inconceivable that the fighter and cleric have good enough ACs that the enemy is only hitting like 20%. In that case it makes sense for the cleric even in battle to heal the fighter if the fighter is down far enough that he could be one shotted dead or even dropped below zero. So if he is down to 3 hit points then it's reasonable the cleric would heal the fighter only to keep the fighter attacking which is better than the cleric doing so in most cases.
This is where you and I disagree. Playing the PC cleric, I would almost never heal the PC fighter at 3 hit points unless he was swarmed by foes or some such. It would be rare for me to do so. It's a waste of my time to do so when I could be taking out or seriously wounding a foe instead.
I typically do not find that a reasonable time to heal the fighter, especially with the 5E healing rules (a better argument can be made for the 4E healing rules).
By healing, I am:
1) Using up a heal resource that might not be immediately needed.
2) Using up a heal resource where I do not get the bonus extra healing of the negative hit points that are ignored in the attack that drops the foe.
3) Not attempting to damage a foe with my action.
4) Having one fewer heal resource in case some other PC falls unconscious.
5) A dropped PC can somtimes be healed after he drops without losing his action for the turn.
6) And one of the most overlooked and important reasons, not healing your fellow PCs in combat will teach them to be a bit more cautious. If players know that the PC Cleric is stingey with in combat heals, they will do fewer stupid things. It's one thing to be heroic, it's another to do something totally stupid because you know someone will heal you if you screw up.
By healing, I am automatically not using my action to damage the enemies. It's not the same as losing my action for the turn, but it's not that far off from it.
Yeah, healing in combat pretty much sounds suboptimal in the vast majority of scenarios. 4E has trained players to heal a lot. 5E is a different set of mechanics with different optimal tactics.
I would say though that short of really needing to heal someone that you would never do it in combat. Cure Wounds gives you more bang for the buck right? So just wait until the battle is over if you can.
I am only disputing the notion that healing in combat is a trap option. A trap option is one that is bad. It's not good 30% of the time and bad 70% of the time. If it is situationally good then it's not a trap option.
This seems to be a bit of semantics. From my POV, healing in combat that interfers with the healers other actions is something that might be down 10% of the time. Or 20%. It would be the exception instead of the rule because it will typically waste more resources than it does saving resources. Hence, suboptimal most of the time. If you are insisting that the word trap means that it is always bad, then I guess we should use the phrase often suboptimal.