But...if the players don't perceive the Threat Pool as something that will create harder obstacles for them later on in the adventure, then where is the incentive to not buy 3 extra dice for each and every task throw--the Threat Pool be damned?
I think you're missing my point.
My point here - the point you quoted - was that if the GM chooses not to narrate or establish an in-setting facet to Threat, then yes, it will be purely a metagame concept. But it doesn't have to work that way.
In Mutant Chronicles, Threat is Dark Symmetry points. In that setting, humanity is opposed by an malign extradimensional force, the Dark Soul, which exerts its will upon humanity through its servants and through the force known as the Dark Symmetry. Dark Symmetry points do have a connection to the setting, as they can be thought to represent the malefic influence of the Dark Soul upon the world, as it rises up to subvert and corrupt humanity. It is a fundamental oppositional force for the game.
In Infinity, Threat is Heat. Infinity has little in the way of supernatural elements, but the setting is one that emphasises covert and clandestine activities. 'Heat' is a representation of unwanted attention (from local authorities, from enemies, etc), and of the potential for plans to come undone and ordered situations to become chaotic. In the stealth mechanics that form part of Infinity, Heat has a direct relationship with the enemy's awareness, as their ability to respond to characters moving covertly is tied to the Heat pool (it requires Reactions, which require Heat to be spent - lots of Heat means enemies that are highly aware).
Similar relationships exist with Threat (as has been explained at length), that allow the GM to narrate and establish the relationship between Threat and extant peril within the game.
BTW, how would Threat Points be used to advance the narrative without creating additional/increased obstacles for the party?
At the simplest level, Threat can serve merely as a parallel for PC resources. In Mutant Chronicles and Infinity, PCs may have several Reloads for their weapons, allowing them to unleash withering salvoes of gunfire. NPCs don't track Reloads individually, but spend Dark Symmetry points/Heat (for Mutant Chronicles or Infinity, respectively) instead (the GM has one resource to track for all NPCs, rather than a few for each NPC). Similarly, just as PCs can buy extra dice for tests or make dodge and parry response actions by paying Threat-equivents, so can NPCs do those things by paying them.
At this level, there's no scene-editing, merely a
-for-tat parallel to what the PCs are naturally able to do.
If a DM in a D&D game starts increasing the Hit Points of a bunch of enemies that the PC's are fighting just because the PCs are doing a good job of whipping up on them, that's usually considered bad form and bad gamemastering. The players feel as if the DM is trying to "win" rather than impartially govern the game.
And, the players feel as if, no matter what they do, the DM will just make enemies and obstacles harder.
Players lose that feeling of achievement when they do win, and even then, it was only because the DM decided it was time they won (otherwise, he'd have changed the encounter as it was played, making it harder)
And, in D&D, they'd have a right to feel that way... except that you've already brought up the GM's ability to fudge undesirable rolls, and this is essentially the same.
In that instance, the GM is doing so arbitrarily, stepping outside of the rules to make a judgement upon it. That's little different from the old "rocks fall, everyone dies" notion, that the GM can choose to wipe out the group at any moment if he wishes...
In 2d20, the situation isn't identical - the GM is wielding an open, known, and limited resource to make these changes. The players can see how far the GM can tweak things on the fly, and can contribute (or not) to his ability to do so. Spending a point of Threat to add an extra d20 to a Frost Giant's attack isn't an arbitrary change, make outside of the rules (as fudging the roll would be), but one that the rules already assume will occur, and which is limited and regulated by the rules.
There's the difference - it isn't "the GM wields his godlike powers to change the world however he wishes", but "the GM is using the tools provided by the game to be a participant in the game, albeit one with a different role".
Now, I'm being bombarded with responses and replies, so I'll stop here, gather my wits, and then push on...