I think I see what you're saying.
I understand that the cells themselves do not house the 'raw data' but are aggregates. Still, I'm hoping for some way to edit the cells that compose the aggregate.
Double clicking a cell would pop-up the table of cells that compose the data aggregate. this could be done recursively until you hit 'raw data' in which case a double click would allow you to change that certain cell's value.
I still think there is benefit to selecting even aggregated cells. This way, due to call back functions associated with this sort of functionality, at the very least the end user could devise some sort of edit form, or further desired functionality using the selected value.
Editing aggregated cells data aside, please make a point to bring up the versatility that would come with having single + multi/cell selection (along with the call backs associated with them) in the RadPivotGrid.