Aside from the few hundred lives vs. a few thousand lives if the plane was actually hijacked and attempted another 9/11 style crash into a metropolitan area there is another reason you might want a chase aircraft - obtaining camera footage of the plane should the threat prove genuine and the plane is blown up. I would imagine such data might prove quite useful to the resultant investigation in terms of narrowing down where the bomb was located, how powerful it was, whether it was carried on or checked, and potentially even narrow down who might have got the bomb onboard.