This feature could work if it was enabled for subject areas. This appears to be an option only for datasets. My use case is with a dataset and a subject area.

"For a workbook — you can control the Blend behavior" Is this a new feature? When blending a dataset and a subject area the only options are "Extend a dimension" or "Add facts". These are set automatically. The only other options are the join fields. Yes, we can use datasets and dataflows to accomplish specific reporting…

It would be useful to be able to choose the join type when the dataset is the primary driver and subject area data is used to enhance it. Left join would be fine if we could define the primary data source.