While online streaming may not count towards the Arbitron books it can still be profitable and beneficial for a station to stream. In order for listeners to stream they have to go to the website, which drives up the hits to the station's website, meaning more revenue from website ads.
Clear Channel stream sites are also linked with their social networking site, which also helped to drive up hits on the site. Also, CC is selling ad space and online only ads to play in place of the commercials heard on air. These ads are synced with the banner ads on the online streaming page to change when the commercial is played. Increasing the amount of money it can charge for ads since its a 2 for 1 deal. Banners and ad time.
Its taking time for the move to become extremely profitable, and obviously its no where near as profitable as the station itself, but it can still make for a pretty penny, not to mention it increases your brand awareness, which isn't a bad thing.