The following is mostly a mixture of typing as I think and thinking at 3am, so hopefully it comes out coherent and useful.
From the 'What PCG is' page, you have created this as your base definition of PCG:
"Procedural content generation (PCG) is the programmatic generation of game content using a random or pseudo-random process that results in an unpredictable range of possible game play spaces."
A potential problem with adaptive difficulty as a PCG technique is that rather than allowing the player to explore an 'unpredictable range of possible game play spaces,' the goal of adaptive difficulty (as I understand it) is to constrain the player into the game designer's preferred play space. That is, if you are too good at the game, the game will get harder, so that you progress as expected. The same is true of the 'too bad at the game' case.
Take Resident Evil 4, for instance. Much of the tension, the overall 'experience,' in the game comes from managing a small amount of resources, primarily ammunition. Therefore, if I, as the game designer, want to ensure that a certain scene or boss 'feels' right, I need to make sure that the player has a given amount of ammunition when entering the fight, so that they get the experience I designed for them. By adding adaptive difficulty, I introduce a nice negative-feedback loop which prevents players from straying too far from this 'optimal' play space, by either removing excess resources if the player is too good (and thus has an abundance of ammo) or by adding in resources if the player is too bad (and is running the risk of leaving the game out of frustration).
Next look at Oblivion. The adaptive difficulty in this game, as far as I understand it, scales the monsters' levels with that of the player. So if I approach a tower at level 1, I will fight a level appropriate enemy. Similarly, if I approach that same tower twenty levels later, the enemies will have leveled up appropriately (perhaps the bandits have full plate mail and magical swords instead of cheap daggers and torn clothing). Now, again, this adaptive difficulty serves to focus the game into a very comfortable and safe player space…I will always be able to vanquish my enemies, provided I put in approximately the same amount of effort as I did previously. This adaptive difficulty removes the player's ability to explore the more 'dangerous' parts of the play space in favour of keeping the game friendly and playable. Now, this may have benefits for many players (I personally disliked the leveling world), but as far as resulting in the exploration of an "unpredictable range of possible game play spaces,' Adaptive Difficulty, as I have seen it implemented, seems to be designed to do the exact opposite: to allow the designer to better control the player experience.
On the topic of Left 4 Dead, could you point me to a description of its adaptive difficulty, if you happen to have one? Because my limited experience with the game seems to suggest that what the AI Director is more an example of Algorithmic Difficulty, rather than adaptive….if I try to play a campaign on a difficulty that is too high for me, I will just fail the campaign. Likewise if I play on a difficulty that is too easy, I will run through it unchallenged. The AI Director may place the zombies in new locations each time, or order in zombie waves in different locations, but if I opt to play a hard campaign, then the AI Director will algorithmically give me a hard campaign.
Anyway, I hope I've at least provided a decent springboard off of which to continue thinking about the issue of adaptive difficulty as it relates to this wiki.