Seriously.
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Blender is free, and whatever 3D modeling tool you use you would still have to learn how to use them. Why waste time developing modeling tools when there’s 3D software out there that will do things way better?
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There isn’t.
Unless you’re counting, like, Hammer or Trenchbroom.
*
A level design tool and a 3D art tool are not the same thing** and do not serve the same need*