Don't forget to read the title text! (Hover over the image to see it.)
To me, it points out that people who say things like that aren't saying anything about the value of the arguments of atheists and Christians. They seem to be more concerned about pointing out how nice and accommodating they are, without taking the risk to actually talk about and defend what they believe, about a very important question.
And it is important. I became an atheist fairly young (in my early teens), but I continued to ask questions and think about my position. I figured that if I was wrong, and a god actually did exist, I could be in big trouble, according to some arguing for that position. Even though I suspected the whole heaven/hell thing was just a scare tactic to hold up a flimsy initial premise, I thought I should at least think about it long and hard before I commit myself to the opposing position.
I started reading about biology, physics, and eventually cognitive science, because consciousness was one of the big problems I had with the materialist position. How could such an incredible ability, conscious thought and decision making, be explained if there is no supernatural soul, or mind? While I don't have a definitive answer yet, I've found a number of interesting possibilities, in books like Society of Mind by Marvin Minsky, and Gödel, Escher, Bach: An Eternal Golden Braid by Douglas Hofstadter.
I'm not sure either of them are right, or that their explanations are even defined well enough to test, but the point is their explanations are based on reasoned arguments and logical chains of inference. The dualist explanation seems to be mostly based on ignorance; it's a "god of the gaps" argument. "I can't think of a way consciousness could arise out of material alone, therefore it couldn't." (I know there are some more philosophical arguments for dualism, which are more sophisticated than this. I don't want to get deep into these arguments, although they are interesting. Suffice it to say I haven't found them convincing.) I found that although we may not have a convincing theory of mind just yet, I'm much more confident that it can be explained materialistically, and that that is the only explanation that would explain anything anyway (if all I say is the soul gives us consciousness, I have explained nothing about how it works and why it should work that way).
My point is, its important to think about these questions. And the fact that some people are willing to go out and argue their point on the issue is important. We need as many people talking about it, and presenting new ideas and arguments. The people who only complain that atheists and Christians are annoying, are mistaken in thinking the important thing is to always be civil and accommodating. Rational thought and argument are important for getting at the truth. That doesn't mean we have to be rude and condescending, but more often than not, these complaints seem to stem from the fact that atheists are questioning another person's beliefs, regardless of tone.
So to all of you who feel this way, try stepping out on a limb and talk about/defend what you believe. I know sometimes people get mad at you for it, but it's important.
(Wow, I didn't expect to write so much about a one panel comic this morning on the train. I suppose that's why XKCD is my favorite web-comic: it inspires deep thought, starting from a simple observation. Or it's because I'm a big geek. Either way...)