@JonKemp It does seem to make perfect sense that sum/count/average should be included in the viewer license. I’ve logged it for future consideration. The request to export has been made by other customers too, and is already logged.
Otherwise @aaron.roma and @Steve_Shuman are correct - the main benefit of a viewer licence is that restrictions will apply, which can be both a time saving and security feature.
Viewers also get a Phocas homepage with potentially many favourites and dashboards. External links are individual and there is no home page or login.