Professor Phobos said:
I am grumpy by nature. Pay me no mind.
But I am interested in seeing your reasoning because I cannot follow it. You have said that this interpretation of the rules (that they do not model the entire game world) leads to an unfavorable outcome over a sufficiently long game, yes?
I would like a step-by-step walkthrough of this process, because I can't follow you in the abstract.
Let's assume that there is dissonance in the players' interpretation of the world. (See my previous elf dex mod example for one such). If assumptions are not shared, you'll run into conflict when players attempt to act in ways that make sense according to their understanding of the world, but don't according to others. You, for instance, find it irritating when 12th-level fighters in full plate cheerfully swandive off cliffs, secure in the knowledge that rock from a great height is insufficient to kill them. This violates your own expected reality; namely, that the world works in a way such that high falls can kill anyone and shouldn't be taken lightly. However, many people interpret the world differently; heroes past a certain level stop having the expectations of reality applied to them. A focused 20th-level barbarian, for instance, can engage in feats of strength that not only exceed what any human has done, but violate our understanding of what human flesh and bone can withstand.
The default understanding of the D&D world is that the heroes aren't just well-trained ordinary people who keep getting really lucky; they actually can do things and withstand hazards in a way flatly contrary to what ordinary people can do. If you understand that people are people, they get lucky, but sometimes they don't, then it makes sense for hit points not to be applied in certain cases; luck (in the aspect of the narrator of the story) decided they didn't. If you understand that regardless of specifics, certain hazards just don't have enough oomph to singlehandedly slay heroes of a certain caliber, and that a fall from a horse is defined as not having enough oomph to do so, then treating a high-level like a lucky ordinary by killing him with a fall from a horse violates narrative expectations. This isn't a problem that can be resolved with flipping a coin or taking turns; it's a question of what kind of story we want told.
The rules, if nothing else, make this perfectly and explicitly clear. The rules define a game in which magic (or at least arcane spellcasting) is a known and understood force. They define a world in which a dozen armed and armored men are no more than a brisk morning sparring match with a legendary hero; not because tales of the hero's prowess are inflated, but because he actually can withstand a dozen men slashing at him and come out without major injury. It's a world in which every creature has a quantifiable amount of life energy; you can increase your amount through experience and great deeds, or decrease it through ripping it out of yourself. It is a world where luck itself is a measurable, quantifiable force; bonuses and penalties based on it can be directly applied and directly sensed; beyond that, there is no narrative expectation, only naked chance.
You can, of course, modify any of these expectations and still be playing D&D. However, doing so without modifying the rules in advance means that, if any of your players hold any of the above expectations and are counting on them to present enjoyable game elements, you will create conflict. At best, you'll send mixed signals, creating confusion ("Spellcasters with total mastery of the ten levels of magic can't summon an efreeti with a second-order spell. If I can replicate how my apprentice performed this feat, I will revolutionize arcane magic in the lands! I must drop everything to research this new and exciting development!"). At worst, you'll send the players into learned helplessness, in which they feel the need to confirm with the GM every nuance they try to read into the world, lest they mistake a dramatic flourish for a meaningful in-world event.
There are a certain percentage of players who will up and hit you with the PHB before they get to the worst-case scenario. If you get a player who requires a consistent world to have fun playing, who demands that the story take place within the boundaries established by the world and not the other way around, who will balk at having his character surrender with a knife to his throat, because he's been in three dozen fights by this point where someone tried to stab him in the neck in combat and failed and fully expects to be able to dodge out of the way or suffer at worst superficial damage. (Side note: if said hero is being threatened by a rogue or martial adept, he will be for a painful surprise.)
Having the rules laid out clearly in advance prevents this kind of misunderstanding. It gives you a common language to describe narrative expectations, so a characters who expects falls to be insta-death or crippling injury 95% of the time but is fine with the idea of walking through a dozen grazes and near-misses in a high-caliber firefight doesn't bog down an exciting scene renegotiating expectations.
Finally, there is my own personal experiences; GMs who tend to "Random stuff happens because it's realistic." tend, in my experience, to not be modelling anything particularly realistic. (As mentioned up-thread, it's actually a bit difficult to slit someone's throat.) If the GM really wanted to model realism, he'd come up with a rule that could be generally applied, and apply the rule. If the GM wanted a specific, non-repeatable event to occur, he should use a specific event (say, the knight happened to ride over an irate elder earth elemental passing through the Prime on his way to Sigil.) Doing otherwise strikes me as unwillingness to tell a story within the world presented.