The other big problem is lawsuit exposure. Given the current state of the patent system it's certain that nVidia drivers infringe on someone's patents.

Closed source drivers make it more difficult for patent owners to check, and launching a lawsuit without evidence of infringement is very expensive.

They claim that their shader compilers/schedulers are part of their performance benefit, yes.

But they don't need to provide that source, just hardware specs. Gallium3d already has shader compilers and schedulers.

But these code bases aren't nearly large enough for it to be a problem for a reasonably sized team armed with decent tools to disassemble and analyze them if the goal is to figure out the techniques used. It might be their reason, but if so it's a damn stupid reason.