Implementation of surface mesh rendering is certainly easy and photorealistic rendering is widely available in various software applications. If creating of surface mesh (=model node) is not a problem then of course this approach may work well. Surface mesh files can be generated directly from segmentations using “Export to files” feature in Segment Editor.
However, most often creating a surface mesh requires significant effort and many cases (e.g., CT volumes) a number of structures can be displayed directly by volume rendering.
Anyway, if we decide to expose VTK’s photorealistic rendering engine (OSPray) in Slicer (see here) then it will be usable for both surface rendering and volume rendering.