The bumpmapping calculation needs to be done in texture-space for some reasons. If not, you would need to make the normal map in world-space (or any other), you coundn't animate your models, ect. For level textures this means that you would need a different normalmap, for every wall that faces an other direction.

For building the matrix which transforms your coordinates to texture-space you need the vertex' tangent, which isn't avaiable on level geometry, at the moment.