Why I can't set data after Mesh.UploadMeshData?

As I understand markNoLogerReadable removes vertices, normals, indices, etc. from system memory. So, those data are not longer readable.
But why I then can't write new vertices, indices or etc?

I thought this is a bug, but not: https://fogbugz.unity3d.com/default.asp?823039_h3sh4lpqebsfbguv