The dangerous thing I realized from the Young Wizards books was this: "Wizardry does not live in an unwilling heart."
It makes it sound as if wizardry simply evaporated when the heart or mind was no longer willing to bear the concept of wizardry, leaving, at worst, a nameless sorrow at the bottom of the soul, of the sort that Nita would have had if she'd ditched out her obligation to the Song of the Twelve in Deep Wizardry.
And that's how it may be in the Young Wizards universe. It's an author's privilege to write things as they should be in the universe, not as they are.
The books do cover what happens when a wizard is touched by the Lone Power and goes bad or mad. And that's as so. But in the real world where things aren't so tidy, there's a vast area between believing in the wizardry enough to do things with it and disbelieving in it enough and painlessly enough that it simply disappears as if it had almost never been there. Times it happens that the power, and the potential, are there, but the heart is so vastly unwilling or twisted that actual proper wizardry is nigh unto impossible. When inadvertent wizardry slips out around the cracks, the heart grows less willing to see it, for wizardry is impossible, after all. If an active wizard on errantry comes by and attempts to shake things loose, the results can be worse than Nita facing her Seniors telling her it was all RP, ah, wasn't it fun, little girl? Now go grow up... Much worse. Much, much worse.
And make no mistake, the wizardry will attempt to out. And it doesn't just come as bidden after taking the Oath. Sometimes it's born in. Sometimes it's woken far before any Oath. Sometimes it takes even a half-joking Oath as true and comes on full after the Oath is forgotten in childhood. Who could dream that the fantasy tales you played with your sister as a child could have repercussions in the Real World after you supposedly grew out of them? But no matter how it decided to arrive, it's there, and it's leaking around the blocks and baffles set up in the unwilling heart to prevent accidental magic. And every now and then, accidental magic happens, sometimes with personally or psychologically disasterous results.
In a makinglight thread a good long time ago, a thread started with two dreadfully clue-negative would-be Darwin competitors horsing around with makeshift lightsabres composed of flaming gasoline in fragile glass tubes, someone gave a well-thought "proof" that magic does not exist: namely, if magic did exist, there would be all sorts of people trying really damn stupid spells, and the results could not be disguised as any sort of mundane injury.
I had to laugh. I'm the sort of practitioner of magic who makes it my sworn duty to help mop up after and prevent beforehand just that sort of problem. Mercedes Lackey makes it very clear that Guardians-as-she-writes-them are a fictional entity, made up out of equal parts chivalry, fiction, and decent pagan-grouping research. On the other hand, it's a very useful and tidy way of phrasing something that's equal parts job description and Calling. On the gripping hand, even if she hadn't written about them, something of the like still would have invented themselves. Organization? One might as well herd cats, or computer geeks, or pagans. Call it coalition. Like attracts like, and can be persuaded to stay in loose contact every now and then. Hierarchy? 1337-spiffy magic-users? Competent and self-selecting in networking purposes, rather. And people who get dangerously riled up at the thought of being ZOMG EXCLUDED from any sort of group that may or may not exist that they want to be in are the sort of people who anything calling itself Guardian ought to be guarding against.
Magic as it exists is far more psychological and intangible than fantasy-based magic. Gods and demons work through the physical world, in the little corners left to them by chaos and psychology. That accident-prone drama magnet may well be victim of a self-inflicted Stupid Magic User moment, much like those Darwin competitors were left with nasty burns. It's a lot harder for the competent mage to hand out violation tickets on the highway of Darwin-Potential Magic when the Darwin-Potential Mage firmly believes that magic does not exist. At least you can whap would-be flaming gasoline-in-glass lightsabre duelists over the head with a goddamn physics textbook and arrest them for Doing Really Stupid Shit. It's a lot harder to call an end to the insanity when the person who just magically caused all manner of panic and disorder with a nasty emotional vortex and a 5-point blow to the love lives of all the people within a shout's range is flatly denying that they just did this and that the magic to do it at all, much less sense it, exists.