When modeling things out that can't easily be build with the SMT builders I find it beneficial to still set up Smart Layers for part naming and material assignments. When modeling completely custom 3D solids it would be great to assign Smart Layers straight to those solids. Currently, you can only apply Smart Layers to specific 2D entities (polylines, etc.). My typical workflow is to have a 2D polyline that I can assign the Smart Layer to and then I use MATCHPROPERTIES to assign them to my 3D parts. It "works" but it adds steps that would be completely unnecessary if we could assign the Smart Layers directly to solids.