I think this is an excellent point.if healing only occurs out of combat (or mostly out of combat) then that means no combat is actually very dangerous. Or, conversely, you are either mostly healthy or all dead (as is the case in something like Basic/Expert D&D). There's no in between because, without in combat healing, there's no way to make the combat dangerous, without making it very lethal and seriously swingy.
4e's assumption was that every combat was a slog-fest to the death
<snip>
5e's assumption tends to be that combats aren't necessarily crazy lethal, so you can see more attrition over the course of an adventuring day: imagine if all of your surges were converted to HP and then just added to your total.
You make a lethal combat in 5e by giving it the ability to drain the party's entire day of resources at once, making it a true "give it your all!" kind of moment.
Is it possible to discuss things without pejorative labels like "slog fest", "scream heals" etc? No one who enjoys 4e and its warlords and incombat healing reaches for those particular labels to describe why they like it.of activatable, intra-combat healing <snip> completely nullifies the genre trope of the "rally" and especially multiple "rallies" in a particularly epic fight. Under that scenario, going to the brink of death would mean that you either offensively nova (if such resources are available) or you are utterly at the mercy if the dice for the rest of the fight.
Anyway, Manbearcat here captures what is different for me in 4e combat compared to traditional D&D. (I don't know 3E well enough to compare it, but have got the impression from reading others' posts over the years that incombat healing, especially Heal spells, becomes important at least at mid-to-high levels). Namely, 4e combat has rallies that turn on something other than the swing of the dice, namely, skillful play that unlocks healing surges. (It's also worth noting that this function of healing surges is largely independent of their role in rationing out healing over the course of an adventuring day.)
No doubt D&Dnext permits tough combats that require "nova-ing" (per Manbearcat) or "giving it your all" (per KM), but that is not something that 4e can't do. I assume 3E can do that too. But can D&Dnext do the rally? (You don't need incombat healing for that, of course - as Manbearcat noted other forms of death mitigation could do the job, and so could some form of escalation mechanic.)