Actually, I agree with Kerry that we ought not take them out. But not because they would grow bigger. The reason we should not take them out is they are Iran's most vicious enemy, and Iranian nuclear weapons are a bigger existential threat to the USA right now than ISIS, for all of its bluster.
I say leave them where they are, help defend the Kurds, Jordanians and any of our other allies (excluding Iraq, which is now an arm of Iran) they threaten, and force them to expand by attacking Syria and Iran. Once both sides have exhausted their resources, then go in with a multinational force. Do a Dresden on their asses and make the people sick of war, then carve up what is left along sectarian lines, forcing the people to live under marshal law until they are ready to responsibly govern themselves.