Never saying a source is bias. Saying its bias doesnt mean anything, everything is bias. People seem to think if they can contrast the views of a source with their own knowledge, and proving how bias something is, that they somehow stumbled on a gold mine, you havent. As CrOsToWnTrAfFiC said, everything is useful, depending on what you want to know. An English source stating that the German soldiers look like tarantulas and eat little children is unrealistic. So why would this be any use? Because it would be useful to a historian studying English war propaganda.