The reason you can render when there are no lights is that, you can illuminate everything by indirect light.
Try making a model self illuminated, it will light up the rest of the stuff! Or by using the skylight, which isn't a light (in the model sense).
But for this to work it needs to gather some stuff, which is going on in the direct light pass, such as self illumination, opacity etc. However the packing time spend, will not be used on the next time you render, as the lightmaps have already been arranged.
Try making a model self illuminated, it will light up the rest of the stuff! Or by using the skylight, which isn't a light (in the model sense).
But for this to work it needs to gather some stuff, which is going on in the direct light pass, such as self illumination, opacity etc. However the packing time spend, will not be used on the next time you render, as the lightmaps have already been arranged.