For example, there's the on screen HUD that Marek wrote a while back to display stats on the screen. Or there's the Direct3D9 backend. It's unlikely Intel will ever create something like that for their own driver because of legal reasons, but they could have taken advantage of the free community work. Instead, they'll be stuck with the same D3D -> OGL wine translation that the binary drivers need.
Further, if the intel drivers merged into gallium there is a fair amount of cleanups that could be done to the rest of the Mesa codebase. Given that Intel is essentially the only classic driver left, or at least the only modern one that is running shaders.