1. They gain a bit of respect, compared to nvidia, about their commitment to FOSS.
2. It's a way to focus on the latest stuff in fglrx without having to worry about legacy stuff because the legacy stuff is handled by the Gallium drivers.
Many complain that even fglrx drivers are doing a bad job. I have no experience in the matter, never owned an AMD card, but from the experience of a buddy of mine I have to say that statement holds true.