From Daniel Engber at Slate: Wind Chill Blows. Back when nobody read this blog, I wrote about how the heat index doesn't make sense to me, because I know what 95 degrees with "typical" humidity for my location feels like, and telling me it "feels like" 102 is misleading. (That's Fahrenheit, not Celsius; we're not literally boiling here in the summer.)
Something similar is true for wind chill. Both of these measures only take into effect two of the many variables that effect comfort -- temperature and either humidity or wind speed. They assume that these are the only two variables which actually vary -- clothing, amount of sunlight, weight, etc. are held constant. In reality, comfort is a function of many variables, and it's misleading to create an index that assumes it's just a function of two variables. People know that they should take more than the temperature into account, but I've seen quantitiatively unsophisicated people think of the wind chill as some perfect index of the weather.
But let's face it, a wind chill of zero sounds scarier than a temperature of sixteen. (Those are approximately the numbers I heard reported this morning in Philadelphia.) That means more people watch the news.