Some people can afford to work for nothing: people who have already accumulated lots of capital, for example, or are subsidized by their families. For them, “exposure” alone may be enough motivation. But it’s important they do charge for their work anyway. Why? Because the more people work for free, the less perceived value the work has, and the more likely those who can’t afford to work for fuck all will not be able to secure the money they need to live.

Aside from the fact that Heydon and Andy have published a very interesting work regarding the art of (writing less) CSS "Every Layout" - and want to get paid for the value this will bring to your arsenal - this blog post is a very good read.