How so? If that were true you could just simulate a normal map where everything is (0, 0, 1) and be done with it :-)
I suppose you could do this, instead of loading 1 out of those 2 normal maps that are used in that example, personally I think this idea of yours has merit, this way you can save on texture bandwidth. But this is besides the point, whether you use a straight blue texture or just define all the unprocessed normals in the fragment processor as all (0,0,1). is irrelevant to what the shader is ultimately doing.
The nVidia sample is using a normalization cube map, it's this map that will be helping to adjust the light for the contours of the model. This cube map is a world space normal map, it's not (0, 0, 1) like the tangent map. But again, you are absolutely right, the tangent map is useless if there is no intention to add extra detail between the vertices of a low-poly model. It's just that this code example is set up this way. Your idea will optimize that shader for cases like this where the bump detail is not needed.