..one of the reasons is also respond from 3Impact tech guy who told me that also, regarding my question about lightmapping (they using 2 identical meshes for difuse textures and lightmap), so i asked him about polycount and performance drop, and his answer was that actually every engine out there basically rendering same geometry 2 times, if base geometry can store let say 2 UV map sets, so for each UV map set, one render pass...so i was a bit confused...