let me put it differently: it can't cast shadows based on its source and therefore light.  iirc, it casts shadows as if it were an infinite light.  so you could kind of sorta fake the direction of light in an outdoor IBL.  not so great if you were using the IBL for an indoor scene, or anything with multiple light sources.  imho, that's pretty much the same as not doing it, because it doesn't cast shadows in the direction and intensity that it lights.