It messes up the shadows so they don't match the model because the shadows can't be Truformed. Which, in my opinion, is a silly reason not to use Truform on the models. I mean, in the past we had little circles passing for shadows, so at least this way the shadows would more or less look like the models (and no one is going to be looking at them that closely anyway).
That's the problem with the shadows though, not with normal maps, or does normal map have something to do with shadow ?
Does anyone ever done a demo on this ? Just how bad would the shadow and self-shadowing look, with Truform anyway ?
I mean self-shadowing on relatively low polygon model, look pretty odd in itself.