The "Why be good without God?" argument is one of the most oft-heard, and is likely the major cause of the distrust that many religious people have towards atheists. It's hard to see why it's so popular, since its flaws seem so obvious. First, the lack of correlation with observed reality (atheists do not steal, kill, rape, etc. at a higher rate than believers). Second, that even if this supposed moral benefit did correlate with reality, it would still fail as an argument for the actual truth of their beliefs, since the effect would come from belief itself, independent of whether it's true or false.