"How does it differ from using a gray infinate light with a low shadow setting?" Infinite lights are directional. They don't light things from all angles like diffuse IBL does. Whatever is not directly in front of an infinite light, just like any other standard light, doesn't get lit at all. Diffuse IBL is basically allowing you to light every angle of your figures/objects, so you can just use a solid color to establish a minimum lighting value... something other than black. I don't know if a diffuse IBL light will help you in exactly what you're looking to accomplish, but it IS excellent for softening a lighting situation if are looking to rid the scene of any harsh "solid black" areas. It can definitely aid in estabilishing more natural scene lighting.

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Tools :  3dsmax 2015, Daz Studio 4.6, PoserPro 2012, Blender v2.74

System: Pentium QuadCore i7, under Win 8, GeForce GTX 780 / 2GB GPU.