I'm pretty sure that when the history of health care reform in America is written, Megan McArdle will get a footnote as the last person in the country to understand how any of it actually works.
The debate over the "rationing" of healthcare has come down to two sides: conservatives who reflexively hate government and so live in fear of some government bureaucrat rationing off healthcare, and those of us who realize that healthcare is already effectively rationed off by insurance companies. There's no way to have healthcare without it being rationed in some way, because it's a finite good that dispensed by people who know a hell of a lot more about what works and what doesn't than most of the people seeking it. However, McMegan has figured out a way around this, in her own inimitable style - just accept that it's true, but that it doesn't matter, because of markets and such.
But there is also a real difference between having something rationed by a process and having it rationed by a person. That is, in fact, why progressives are so fond of rules. They don't want to tell grandma to take morphine instead of getting a pacemaker. It's much nicer if you create a mathematical formula that makes some doctor tell grandma to take morphine instead of getting a pacemaker. Then the doctor can disclaim responsibility too, because after all, no one really has any agency here--we're all just in the grips of an impersonal force.
But this won't do. If you design a formula to deny granny a pacemaker, knowing that this is the intent of the formula, then you've killed granny just as surely as if you'd ordered the doctor to do it directly. That's the intuition behind the conservative resistance to switching from price rationing to fiat rationing. Using the government's coercive power to decide the price of something, or who ought to get it, is qualitatively different from the same outcome arising out of voluntary actions in the marketplace.
So, when your insurance company decides to deny you a pacemaker based on a flawed "preexisting condition" metric, or cost, or just because they've decided you've exhausted your benefits or because something was coded the wrong way, it's a "voluntary action in the marketplace" based on price. When the government decides to deny you a pacemaker based on price or efficiency, it's by fiat, and therefore bad. Even though it's the same thing. And even though in the government system, you can still get private health insurance. And even though when you're denied by your private insurance company, you're essentially fucked anyway, because nobody else will cover you based on the prior denial. But still, it's okay, because you voluntarily purchased that private health insurance, even when it's attached to your job, and even when you don't get the benefits you were clearly promised, because you were just killed rationally instead of by some government drone coming up with wacky formulas that get you a Nintendo DS and a bean pie when you need a new liver.
It's not so much that McArdle is wrong. It's that she proves herself wrong and then stands on her mountain of stupid wrongness sneering at everyone who's noticed that she's gloriously fucking wrong about everything.