Fglrx already uses a framework similar to G3D internally. They wouldn't gain much from switching over. They could re-use G3Ds state trackers... but they already have their own, so why replace them?
Switching to G3D might make sense if AMD could use G3D for both their linux and their windows drivers. But I doubt that's going to happen soon, G3D isn't mature enough yet. But even if it was, I'm sceptical that the initial costs would ever pay off.
Besides, G3D doesn't magically solve the problem of kernel and xorg ABIs.
There's quite a few people at AMD that know way more about their drivers, their customers and their markets than we do. If it would solve their problems, they'd already be doing it