When creating a DrillThrough report, all of the parameters currently need to be passed or have a Default value. If a parameter has no default value and is not passed in, an error message is shown: "The 'parameterName' parameter is missing a value."
However, due to some business rules, it may be necessary to only pass some parameters and have the user specify the other parameters (no default values) when drilling down. In such situation, it is better to have the default "Specify Parameters Values" instead of the error message shown.
This is because the report is actually functioning as it should be and the error message simply turns off the users, making them think that the report is not functioning correctly when it actually is.
This is using Report Builder 3.0 for SQL Server 2008 R2 on SharePoint 2010.
For more info: http://social.msdn.microsoft.com/Forums/en-US/sqlreportingservices/thread/cc6da803-98bf-4fd0-b5ae-2ce43b336817/