Kevin - this is an age-old debate. Many people refer to DEMs as "2.5 D" since they're clearly more than 2D, and yes, useful for some 3D analysis such as line of sight/viewshed.
The fundamental reason we don't consider them fully 3D is that you can't represent any overhanging features (e.g. look under tree canopy to see the tree trunk). In the case of a building with overhanging eaves, the DSM shows the building as having the full XY extent of the roof footprint, whereas the foundation of the building is smaller - and in most applications seeking the building footprint, they want the foundation, not the roof extents. But for some specific applications, you're correct that the DEMs can be considered and used as 3D datasets.
Personally I think it's an advantage to call these "2D" for users of Drone2Map Standard - the Standard license does not include full 3D functionality but it IS able to create the DEMs (DSM and DTM) - so it's a benefit to these users who may want to calculate volumes of stockpiles or excavations - but the terminology is certainly open to ongoing debate.
Cody B
