Although many journalists have the idea that truth and knowledge is power, I do not believe that everything should be completely exposed to anyone who cares to read it. Confidentiality is not something that can or should be broken for any conventional reason. Privacy is a fundamental right in our US constitution.
If a source did not feel confident that their identity would be protected, they would be much less likely to come forward with information. Some stories are important and need to be told, and the sources for those stories should feel comfortable revealing things to the journalist. Their could be specified cases when that confidentiality could be overridden by the law, for example if the information is necessary to prevent someone's death.