I would love to see this functionality. This issue has come up multiple times for our end users.

As an example of a business use cases, we have a subject area that users often join their own files to. Sometimes those files may contain values of a joining dimension that do not exist in the subject area because the associated fact they are pulling from the subject area has a value of 0. Despite it being 0, they would still like it to be present in their visualization, so other users know that it exists. Because the blending between the subject area and XSA file resorts to an inner join, the row drops out entirely. In order to have ALL values of the dimension shown, regardless of whether they're present in both files, these users need to be able to control the blending options and set the XSA file to show 'All rows' instead of just 'Matching rows'. See attached images for the example I just described (values have been changed to protect sensitive information, but this is a real example).