@AD_Kar, the issue with using a light map rather than a shadow map is that you really need to render HDR tones for the light map to be useful. A standard 8-bit image is fine for a shadow map, but for a light map, you never get any tones over 1.0. They do mix with the lighting in the scene, so the combination of light map plus punctual light can get you to a value over 1, but needing to rely on a punctual light to get your lightmaps brighter defeats the purpose of the light map, especially when using PBR materials.
I am going to point you to this thread which is a very interesting conversation about how to convert an .hdr light map (saved as an hdr from Maya) and converted to an RGBD format. There are a couple of playgrounds in the thread from @sebavan and @Evgeni_Popov that should give you some information about how to convert an HDR file to an RGBD and still get hdr values in your light maps.
You just need to make sure the light in the scene is actually rendering out HDR values (you can check the render in any image editor) and save your render as an .hdr (or an .exr which will need to be converted to an .hdr) and then use the playground in the thread to convert your image to an RGBD file so it can be loaded as a typical LDR file. This will give you the most control over the lighting in your scene from a high-quality ray-traced source.
This technique is something that I see a need for some investment in for the engine so what you are seeing in the thread is the beginning of the process. Hope this helps clear things up, and you will see the samples in that thread do a good job at illustrating the need for light maps to be HDR.