...and into the fire.
Agreed: There seems to be merit in having a contingency option, but the details make this a very questionable proposition. In particular (and as you pointed out), I'm not sure that we can objectively define what sort of attack would justify delaying elections. Nor am I sure that we should even try, since this would provide would-be disruptors with the specifics they might need. Even if we frame this provision in a "non-partisan" manner, that's a lot of power to put under subjective control. We might take steps to minimize that risk, but the stakes cannot be lowered. So while we might be vulnerable without such a contingency, we must be cautious to avoid a scenario that's potentially worse.
(For what it's worth -- as unscientific entertainment -- CNN.com's current Quick Vote asks, "Should Election Day be delayed if there is a terror attack near that date?" Of 162,000 responding, 82% say "No.")