actually, the main thing i'd want to be a light source is specific materials.  like Stonemason sets that have lights defined by materials.  like futuristic touch screens.  like neon signs outside buildings.  i realize most flames and magic effect stuff would be impossible as true lights because they use transparency, but i'd think anything that uses a whole material zone would be fun to use that way.  the main time i'd use a simple primitive would be to duplicate studio lighting, or on a lighting prop that hadn't been made with bulb geometry.