It better be. they can mortar the airport quite effectively from Abu Graib. Cut off the airport, you have a real siege, save the route south, which is not that secure. About half of the 8 or 9 million residents of Baghdad are Sunni
ISIS doesn’t have the physical numbers to occupy Baghdad, nor do I think they’d want to do anything but bypass it, concentrating on their dash to the Gulf. OTOH, if there’s support in Baghdad, it would make a nice buffer and anchor. ISIS found Kobane too costly to hold, but is starting another attack, they have a sophistication to their movements. Kobani was a bad gamble for them, they took serious losses, not least of which was their air of invincibility.