I think I may have the answer, and it's fairly simple (not short to explain, but simple).
I don't usually say or think that, but in this case I think I might. I've only read up until page 11, btw, so this might have been addressed somewhere already...but I'm pulling from a quote on either page 3 or 4:
I don't agree that it "warps reality". If I give an undefined value definition, that isn't a warping but rather a definition of reality.
The dozens of examples and parsings and defining and re-defining and clarifying of terms from page 3 or 4 through 11 (and likely to this point) all dance around this.
Let's start with the value being defined. In such a case, if the player changes it, they are warping reality. If the distance on a map is 20 miles, the distance is 20 miles.
If the value is undefined, the player
might influence it. Are there elves in the marketplace? The DM doesn't know and hasn't defined the value (e.g. this is an elven village or this is a village that is xenophobic about elves) then there needs to be a determination.
Now, the solution:
We are talking about two play style variations, but with a third division in one of these styles.
Style 1: The world "exists". The players do not change defined values. If the value is undefined it is determined not by "it's good to say yes to players" but by what makes sense, perhaps including die rolls to determine. In this style, there is exactly the same chance of an elf being in that marketplace whether the players asked or not. They (and the dm) wouldn't have known about it if it had not been explored, but the CHANCE is the same. In this case the players do have narrative
impact (they determine that there might be an elf in the
story), but not narrative
control (they don't influence the chance for the elf to be in the
marketplace). This is the style I believe @
JamesonCourage 's group uses (as does my own when sandboxing but not when playing adventure paths).
Style 2a: The world doesn't "exist". The story and adventure are paramount. Defined values (the map says 20 miles) remain defined. However, when players get inventive with the story by asking for additional details (is there an elf in the marketplace)
the DM leans toward saying yes. In this case, players have narrative control (it is contingent on the dm allowing that control, but they do have it). There is a greater chance of the elf being there because they asked and because the dm is handing over some of the story to them. The more likely the DM is to say yes, the more control the players have (and the greater chance that the elf will conveniently be there). They cannot explore an "existant" world in the same manner as style 1, because they have the power to impact the reality of that world. This style is, I believe, the most common style, and the one put forth in the "say yes" or "say yes, but" articles by WotC.
Style 2b: The world doesn't "exist". The story and adventure are paramount. HOWEVER defined values do not remain defined. This can be as simple as "the adventure was supposed to take place in the town to the east, but since the players went west, it's going to happen there, the players don't know the difference". It can also be more extreme of a retcon wherein the story is more exciting if the map is only 10 miles, so it becomes so.
To sum up:
There is something gained (potential story directions) and something else lost (potential world reality) when the world conforms to player controlled narrative. (If there is always conveniently a chandelier to swing on, or a torch to grab off of the wall, or a loose cobblestone right when you need it, it can be great adventure, but feels less real.)
A final thought. This also can be an issue with DM controlled narrative on the fly. If a dm decides, "It would be cool if there was an elf in the market, because player x could benefit." it can rob the players of the feeling of reality as well.
I think the sticking point of this discussion is that people are concerned about DM tyrrany, railroading, and other loaded terms. In reality, that's not what the conversation (as I understand it) is about. The conversation seems to actually be about whether cool stuff happens because "it would be cool if" or if cool stuff happens organically, and if so, more rare (and potentially more cool when it does, because it was cool because it wasn't given that extra nudge from any source outside the game to BE cool).
The discussion, while ostensibly about player control of the narrative seems to be much more about the focus of the experience of the game: is the narrative defined by the story or by an attempt for reality?
Either way is fine, but they are different styles.
As always, play what you like.
