it aids in the passage of time. The passage of time allows for more evolution of the setting.
I don't buy the "passage of time" arguement. For one, you're only talking a few days difference most of the time. It's not like 1 week of healing is the standard, it's the maximum (or near enough). Most of the time, you're only healing a couple of dozen HP, so, we're talking a few of days vs 1 day at most. It's not enough time to make any significant difference in the grand scheme of things.
I agree with Hussar on this point, and it's part of why I find D&D an odd choice of system for those who want this sort of thing in their games.
In RQ, hp heal at 1d3 per week, and a character who is healing from unconscious to full health is likely to be healing between 10 and 20 hp. Making for healing times of a month or two.
In BW, healing times for wounds any heavier than light are likewise typically a month or more, sometimes up to 6 months, or even close to two years to fully recover from a mortal wound.
In Rolemaster, even magical healing still imposes healing times, which for a bad injury or a series of injuries may well be more than a week, and in some serious cases multiple months. Natural healing is similarly drawn out.
These are the sorts of recovery times that in my view allow for evolution in the setting. (In BW, this is explicitly called out as one function of the recovery rules; another is to oblige players who don't
want the setting to evolve to get into the action while carrying wound penalties, which changes the dynamics of play both at the ingame and the metagame level.)
But, if you have a game that is 90% out of combat, what rules are you actually engaging? Or, to put it another way, in a game that has virtually no combat, 90% of the game rules are not being used.
I agree with this. I recently reread the Moldvay Basic Rules - which is often held up as a counterexample to most D&D rules being about combat.
Moldvay has detailed action resolution mechanics for combat (surprise rules, initiative rules, a turn sequence, hit and damage rules, morale rules). It has less detailed action resolution rules for dungeon exploration: for listening at doors, for opening stuck doors, for finding secret doors (but not for opening them, which is left to player ingenuity, at least according to the description of play), for finding and disarming traps, and the like. (Some of these resolution mechanics are also hidden in the spell lists: mystical runes and sigils are a staple of dungeon exploration, for example, but Basic has no "read runes" ability, and instead you have to use the spells Read Magic and/or Read Languages - though Expert also introduces a relevant thief ability.)
It has very simple action resolution mechanics for social conflict - there is a reaction chart, which according to the rules is to be used primarily in framing the scene (are the encountered NCPs/monsters hostile or friendly?), but according to the example of play may also be used to resolve it - in that example, after the PCs make an offer the GM rolls on the chart again to see how that offer is received. There is nothing comparable in these social conflict resolution mechanics to the detail of the combat resolution mechanics, and only one stat pertains to them - CHA - whereas 4 stats pertain to combat resolution - STR, DEX, CON and WIS (for saving throws).
There are basically no resolution mechanics for movement (beyond movement rates - but no rules for chases, for example, nor for climbing if one is not a thief, nor for swimming, nor for riding). The expert rules beef these up a little bit, but there are still no rules for (for example) resolving a race.
If I wanted to play a scenario in which the PCs didn't have to find, listen at or open doors (eg they are in an ordinary building in which the doors are obvious and not especially soundproof), and in which the main mode of action resolution was by talking to people, Moldvay Basic would have little to offer. The only relevant stat is CHA. There are no relevant class abilities. And the only mechanical technique the books gives me is the reaction roll table, which is pretty sparse stuff.
If I was clever at such things I might try to adapt the morale rules to make them (i) integrate better with the reaction rules, and (ii) have applicability outside combat. I haven't tried that myself, and so can't comment on how easy or hard it would be. But as published, I think these rules back up Hussar's assertion. Once you go beyond combat and simple dungeon exploration, you're very close to being on your own.