A spec map and an illumination map are pretty much the same, in that they are normally just 1 bit monochrome textures (no reason they cannot also be 256 greyscale or colour either). So you could use the spec map as an illumination map, except that a spec map relies on area lighting unlike an illumination map which does not care about the lighting in the area.
So unless your using one of bond1's shaders with the _I texture encapsulated inside the alpha channel of the _S texture, you will only get the self-illumination effect when viewing the object in direct light and nothing on the darker side.
Plus if you aret using a shader with the _I in the _S textures alpha you are not in fact using a spec map as an _I map. Also if you are using the spec map with a normal illumination shader (i.e. illuminationent.fx etc) you are just using an _I texture that is made from your spec map and this is no different from using an _I texture anyway.
