I think the problem is more that ISIS is an existential threat to the US definition of the region. Right now, we're kind of reliant on the western-defined state of Iraq as a vital balance of power in a region where there are otherwise vanishingly few stopgaps between US allies and... everybody else. Without a clearly-bordered, stably-governed Iraq, US interests are in big trouble. And it looks like that's kind of where things are headed without any further US intervention of some sort. Maliki is a bastard, but he's a convenient bastard, or was before he demonstrated a complete ineptitude for either fair governance or effective autocracy. But he looked so good on paper! Strong academic background, good wartime stats and a history of fighting our enemies while at the same time staying reasonably detached from anti-American actors, and his Shia pedigree gave him both a legit distance from the old Iraqi governing forces and a diplomatic line to Iran.
Without Maliki or somebody like him (and past star searches have proven a marked lack of those), and without the Iraq as we've come to understand it on a geographic level, US regional interests will fall into deeper trouble. And in that respect, ISIS is every bit as dangerous as people have been crowing about.
I like this article a lot, but I'm not sure I understand the basic premise- "ISIS isn't as bad as everybody is saying, they're just really good at de-legitimizing weak governments through show of force!" That's, like, exactly as bad as it gets for all intents and purposes. A government is only as strong as its monopoly on power- or at very least the illusion of a monopoly on power. If ISIS continues to dominate the Sunni-heavy regions of Iraq with impunity- and all things being equal there's no reason to think that they won't- then the Maliki government will crumble. At which point, the state of Iraq either falls back into the hands of those with markedly less interest in the comfort of US regional allies, or else the whole region falls back into an official state of fractious tribalism. Either way, bad news for American interests.
FWIW, I think there's close to no chance that we'll actually waltz into Iraq for round three. Domestic appetite for war is too depressed, our military forces are already stretched too thin in a time that we're supposed to be "pivoting" towards Asia, our budget is too strained to support the cost of another ground war, and Obama's official doctrine has been clearly painted as one of less direct intervention. Drones and air support, sure. Money poured into the Maliki pit, wouldn't be surprised. But Iraq III? Unlikely.