When dealing with sparse data or data blends, or unpivoting to create a very tall data source in order to get the desired layout, a Cartesian join can be useful. A number of example for Tableau are outlined in The Cross Join Collection. On Tableau for Windows, for Excel & text data sources a Cartesian join is possible via Custom SQL. However, Custom SQL is not available for on Tableau for Mac so those users will have to use some other means. I'd like Tableau to natively support Cartesian joins in the new connection interface.
For this work, http://community.tableausoftware.com/ideas/3382 (custom WHERE clause) would also have to be implemented so that the proper filtering can be done of the Cartesian join.