I agree very much… 3D viewer is a very useful tool…
moreover, if you want to export your board to 3D for mechanical purposes or interchange (e.g. if you want to check your dimensions in an enclosure), you may consider to add 3D parts generated from FreeCAD or from STEP models.
Then you may use the open source kicad StepUp script to convert board and 3D VRML models to 3D mechanical objects for ECAD MCAD collaboration.
So, if you need to check if your board can fit in an enclosure, kicad StepUp will give you the answer…
Using STEP models in your design will give you the ability to export all the artwork in mechanical 3D environment, letting you to design your kicad pcb with the same dimensions - appearance that you’ll get in mechanical software.
You can use FreeCAD (open source) do do all the job! 
Moreover, kicad developers are moving to integrate 3D IGES and later STEP format directly into kicad 3d-viewer… so the first you move toward this approach, the less you will have to change later…
STEP (or STP) format is the standard de-facto for CAD interchange
STEP format description
and gives you a mechanical model with real scale factor for pcb enclosure design and collaboration
here a description of the ECAD MCAD importance in PCB designing
ECAD/MCAD Collaboration in other software
here
an example of kicad EDA and mechanical CAD artwork collaboration and
comparison in kicad with the kicad StepUp script, already usable with
the actual RC kicad release…
You can try also FreeCAD 3D models script generator and some generated models, usable out ‘of the box’ for kicad…
just for completeness, here some recommendations for a 3D CAD package for KiCad available in the forum
Maurice