Huh? The lack of GI is not the reason we break the BRDFs into Diffuse & Specular terms.
Specular formulas simulate lighting coming with a particular polarity pattern while Diffuse lighting simulate lighting coming with polarization patterns on a different axis.
This is a real life phenomenon unrelated to Indirect/Global Ilumination.
I always thought that the traditional difference between specular and diffuse terms was simply that the specular term is only the light that is perfectly reflected from the light source, rather than being reflected over some distribution of angles (although I've also colloquially heard specular be considered any light reaching the camera dependent on the angle between the camera and the reflector as well; in this case diffuse would be limited to "ideal" diffuse, and certain kinds of materials like Minnaert shading, which give off light in certain directions but not dependent on the angle of entry, would still be considered "specular"). My recollection of how polarity works seems to suggest that it isn't sufficient to make that distinction, at least in all cases.
In all cases you just need to be careful in how you set up the interactions between the two BRDF terms if you want to maintain energy conservation.
I'm a bit curious about this. Since real-life materials are perfectly capable of absorbing light and then re-emitting it as energy other than visible light, what does it really mean to have a material that doesn't conserve energy? I guess materials that absorb light in especially strange, angle-sensitive ways are probably rare, but it seems plausible that some arrangement of microfacets could potentially be described by materials which are obviously "wrong."