Tableau v8 added a new style of data blending (DB2) where the linking field does not have to be in the view. However, when the linking dimension is not in the view, non-additive aggregates from the secondary like COUNTD(), MEDIAN(), and the RAWSQLAGG_xxx functions are not supported. If one of those measures is brought into a view with a linking dimension not in the view, or a linking dimension is turned on that is not in the view, the pill will turn red, the view will turn grey, and hovering over the pill reveals a "Cannot blend the secondary data source because one or more fields use an unsupported aggregation." message. Though I recognize that getting an accurate computation is fairly complex (and may never be possible for all circumstances, I can imagine some table calculations situations where it would be difficult), at least supporting some cases would be helpful.
One thought is that if Tableau supported something like TOTAL() for secondary data sources, then that could be set up with the right aggregation to return accurate results.