Add Data has a lot of capabilities, which result in complexities. These are not necessary, if you just want to load a single volume or scene. Basing the initial presentation on the add volume pop up, which is optimized for just that, would accomplish this. The option button in the lower right allows to bring up the add data interface with all its capabilities and complexities.
Under the hood, we need to add the capability to load mrml scenes and SDB's through this interface and remove the dicom capabilities, which have migrated to the DICOM widget. See attached mock-up.

I also want to rename this dialogue into DataLoader.