Peter: I think the whole concept of mechanical advancement actually pushes a game away from a fiction-based (as you use the word, or perhaps simulation-based is acceptable) space toward a trope-based or arc-based space. Here's why: (warning, LONG)
Let's start by talking about what supernatural or paranormal elements do to a game setting. Obviously the moment you introduce them, "realism" as it's most strictly defined goes out the window. But there's such a thing as "verisimilitude" and "coherence." You can make a world that *feels* real--you can combine suspension of disbelief with abstract reasoning to imagine what would happen in a world that wasn't like ours: you have to even without supernatural elements in *any* sci-fi setting. You have to be able to "simulate" the effects of economies and technologies that never were to run AW at all. That's verisimilitude. And you can make sure the world works "The same way every time"--that's coherence. And those standard's don't go out the window because you introduce a psychic. You can and should strive for a coherent world even with magic and monsters in it.
I'd argue, however, that something slippery happens when you apply the concept of coherence to magic: a change in what it means to work "the same way ever time." See, with a futuristic motorbike, you can straight-up DEFINE it's gas mileage, it's acceleration, the weight it can pull, if you really want to. And indeed once an MC has told you it takes 8 hours to drive from A to B, it had damn well better take 8 hours the next time unless you're redlining it or the weather is bad.
When it comes to something like "open your brain" though, there aren't any real-world benchmarks to compare it to. You can't write out the mass and power of the Maelstrom, you can't say it's like a cross between a Honda and a Mazda, and you don't expect the results one character got from opening his brain to be reliable or replicable. So how do you prove to your players that you're "playing fair?" First, you give your world's Maelstrom a consistent theme, like ghosts or emotions or secrets or mental disorders. Your players will describe their game's Maelstrom to their friends by analogy, not to real objects, but to other fictional texts. They'll say, "our MC plays the Maelstrom like the underworld from Earthsea" or "like the Force" or "like the First Evil on Buffy" or whatever. Second, you make sure that even though what happens to Marie when she opens her brain may never have happened before or again, it feels appropriate--that after the fact it seems like it should have been predictable.
But that sense of propriety is, I would argue, an arc-based sensibility. We come up with "reasonable" supernatural scenes by modeling them on our favorite trances and haunting from movies, which used them to advance themes or resolve character arcs.
So what does this have to do with the advancement rules? Well, here on Earth I'm pretty comfortable saying that people essentially never suddenly gain a level in badass. Someone does not, from one day to the next, acquire Ice Cold or Good in the Clinch or even a +1 to hot. One especially does not, bleeding and forsaken by all allies in the wasteland when the raiders are coming, set his jaw, acquire NTBFW and murder the lot. (Notice I invoked a trope there). Sudden, dramatic change in capabilities are a fictional trope used to move the spotlight around an ensemble cast or resolve a previously irresolvable plotline.
The playbooks themselves are drama-based objects, created to bestow players with the powers of fictional protagonists for no in-universe reason. A Battlebabe's abilities, or a Driver's, are every bit as supernatural as a Brainer's. And the advancement rule is also tailored for the sake of drama: by giving when dice are rolled (and dice are only rolled when the shit hits the fan), the advancement rules actually assure that PCs usually *will* pick up new moves while they're broken, bleeding, or abandoned. For that reason, I think "when dramatically appropriate" would be a pretty good approximation of advancement as it stands.