Let me be more specific - this is a bug in the import and export of obj/mtl files.

Don't take my word for it - look at line 174 of export_obj.py already in Blender - you will find map_Ke - the keyword to attach a texture to an emissive color. Works the same way as map_Ka, map_Kd, etc. but for emissive light. Only makes sense if there's also a way to change the emissive light color like with Ka, Kd and Ks for other kinds of light.

Blender will also change "illum 0" to "illum 2". While technically a bug, illumination model modes are almost never used so probably not a priority.