The argument that we shouldn't worry so much about terrorism because cars kill more people than terrorists is seductive to faux intellectuals like Bill Maher. The more general point is that we as a society accept all kinds of risks which are greater than the historical, statistical danger due to terrorism. I'll focus on the comparison to cars because, as far as we know today, the largest risk of violent death in the US comes from car accidents.
In 2008 there were 34,017 deaths (and nearly 100,000 major injuries) related to automobile accidents in the United States. Terrorists would have to blow up 113 Boeing 777-200s each year in order to kill that many people! That is, they'd have to blow up all but six of the 777-200's (which hold 301 people in a 3-tier international setup) currently owned by American Airlines, United Airlines and Continental Airlines (together they own 119 777-200s) and would have to do so every single year, which is probably faster than they can be built. And yet there is hardly any talk of defending the American people from their Buick!
The first reason that the analogy is simplistic is that the car is a necessity in modern life. The benefit we derive from cars vastly outweighs the cost. In terms of deaths and injuries, cars almost certainly save far more lives than they end prematurely. There is no benefit to us from terrorism that I can think of, but plenty of costs in addition to the direct damage and fatalities. Second, most car-related casualties happen due to negligence or recklessness, and those casualties usually happen to the people who are directly responsible. One can reduce the risk of traveling in a car dramatically by wearing a seatbelt, driving carefully, and maintaining the car in good working order. Third, society has already come to terms with the risk. This is not something that can be easily duplicated in other areas. Traveling by plane is still far safer than driving the equivalent number of miles in a car, but far more people are afraid of dying in a plane crash than in a car crash. This may be irrational, but it is a reality. Terrorists can dramatically raise the fear associated with flying (rational or not), and if they do that they can degrade our standard of living as they already have done to a significant extent since 9/11.
Finally, it is not clear how effective terrorists can be if they become encouraged by success. The daily dish reader is correct that it is extremely unlikely terrorists could cause as many deaths per year as cars do simply by blowing up planes. But success at blowing up planes encourages more people to join the terrorist/jihadist cause and emboldens the leaders of that cause. It's possible that such success actually increases the probability of nuclear terrorism. Even Bill Maher would understand that a small nuclear explosion in a US city would dramatically change our society for the worse.