Ad rates are continually dropping, so I'm sure the current figure is lower. And you're assuming a random blogger would get the same kind of yield as the mostly professional participants in the ad network there.

And even if you're right, an extra $2k in income would only be nothing to sneeze at if the ROI is decent. But competition for eyeballs is fierce. There's a reason that even professional writers are having a hard time making ends meet.

... and it makes a world of difference for some people.

Ad clearinghouses are a flattening and democratizing force on the Internet that I don't think people recognize as such. They let people who otherwise couldn't really afford to pay for server space put up their content.