The "rules" are a system to help organize and structure play in a fictional universe. They help answer questions like: "I swing my sword at the orc, what happens?" Unfortunately, practical limits mean that the "rules" cannot be designed in such a way to be applicable in all possible scenarios. Even with a 10,000 page Core Rulebook the simulation will break down. Most RPGs employ a DM to use common sense to determine when and to what extent the "rules" are applicable to the current scenario. Rule 0 is the explicit statement to this effect (or permission, if you will). In this sense, RPG "rules" are not rules in the manner of games like Monopoly, Poker or a tactical war game, they are "guidelines" or "tools" that give DMs and players a common framework and language to resolve fictional events in a consistent, coherent and enjoyable way.
In the parlance of design, this is a feature, not a bug. You can play an MMORPG (for example) all you want, but you can never leave the edge of the map. You can never attempt something that isn't pre-programmed into your character's skill set. You can only follow pre-ordained quest lines and goals and never devise your own. In TTRPGs, the DM can, using their creativity and the system as a guideline, determines what happens in these situations. In my mind, this is what makes it great.
I don't see how you could possibly reduce the DM's role to "Arbiter of Rules". Consider this scenario:
The party has defeated the dragon [using the adventure encounter and combat rules] and want to take its treasure horde back to town. Unfortunately/fortunately, the party and their hirelings can't possibly carry the entire pile themselves [using encumbrance rules]. They discuss it and hit upon the idea of going back to the orc encampment they wiped out to grab some wheelbarrows to haul the treasure. The DM looks at the encampment section in the purchased adventure and doesn't see any specific notes of wheelbarrows but the orcs had been using goblins as slave labor to build a massive series of earthworks. Wheelbarrows would be plausible in this concept, even if the adventure doesn't explicitly mention them. Should the DM nix the players' idea, make a roll or just say there is a wheelbarrow there? Let's suppose the DM decides that rolling a 1d6 is the most impartial method. On a 4+ there's a wheelbarrow. The players are lucky and find one.
The players load up the wheelbarrow but its still not enough. They ask if they can drag some of the chest back to town. The DM checks the rules and doesn't find anything about dragging chests but decides that it should be possible (Why the heck not? People drag stuff all the time). He looks at encumbrance rules again and judges that such a set up will cause the group to move at 1/2 the slowest movement rate possible.
It will be a long trek back to town from the dragon's lair and the DM switches to the overland travel section of the adventure, which states there is a 1 in 6 chance of wandering monsters every hour. The DM considers the situation (the overburdened, slow-moving group with a wheelbarrow, dragging chests through the wilderness) and decides to implement Rule 0 by saying they are making so much noise they will attract notice on a 2 in 6 chance each hour, despite the fact that rules or adventure say anything of the sort. The DM judges that the situation that the party is in has deviated enough from the assumed situation in the rules and adventure that it no longer applies.
The DM rolls the checks and, on the fourth hour, a 1 comes up. Another roll on the table provided by the adventure gets 4 ogres. The table describes the band as "greedy and cowardly". Using the standard encounter rules and the party's actions, the encounter starts of as a parlay rather than a combat. The party doesn't want a fight and try to bride the ogres with some of their treasure. Using the greedy and cowardly descriptor of the ogres, the DM decides that this is worth a bonus on their interaction rolls. The party ends up buying off the ogres with the two chests the fighter was tired of hauling around anyway. However, the DM also judges that the ogres have now spied the massive treasure that is within their grasp and decides that the ogres, being cowardly and greedy, will attempt to follow the party, hoping to ambush them in a moment of weakness (despite the fact that nothing in the adventure or the rules says to do so).
And, thus, play continues.
