The ability not to flip UV's with the mirror modifier, but still offset.
At first this option seemed quite strange to me, but its infact fairly common use in games industry it seems.
It allows baking without writing to parts of the image outside 0-1 range. (Blender could support this too).
The advantage with this over baking half the mesh is you get correct normals, shadows, tangents.
For reference see a similar patch: rB31e62492564b32309ce5100ae40a6664b38d7baa