A List Apart has an article about the value of sketching and outlines the need to just get stuff on paper in a tangible form, rather than fussing about how pretty things look or worrying about "artistic" quality.
When you feel inadequate in your sketching, pause and reconsider your perspective. Don’t worry how well you draw. Instead, think of your sketching as visual thinking, which works regardless of your drawing quality. Ugly gets the job done just fine.While sketching can be an essential part of the design process, I think it can help with many different types of work. It can be a great tool to help explain things to people and often the final sketch itself is less relevant than the steps that were taken to bring it about. For example, I remember seeing the video below during the financial madness of late 2008. Paddy Hirsch of American Public Media takes the complexity of synthetic collateralized debt obligations, tranches and credit default swaps and, through the use of simple stick figures and wine glasses, brings it down to a basic level that is more easily understood. The final product isn't a pretty picture (it's a whiteboard, after all) but getting those ideas out quickly and simply is valuable and meaningful. Now I know why I got sacked from Condé Naste!