It's not like a shader that transforms your object space normals with your Bone transform matrix is common practice however. Plus it's a bit heavy on performance.
Is it that heavy? From what i've been told the savings from not having to convert tangent space into world to render makes up for the slight cost of transforming the OS normals. Which means its pretty much a wash performance wise. Also not needing to store normals/binormals/tangents for the mesh gives a nice little memory…