The GPU is the main processor :) The ARM CPU is an addition to the GPU but the SoC is like a BIOS/EFI on standard PC computers.
SoC - System on Chip
The SoC binary is possible closed source because it might reveal allot of technological patents to do with the technology it self. So Broadcom most likely don't just want to open source it to everybody to reuse. It has no impact on the usability of the Pi any way, just like it doesn't matter what kind of BIOS you have in your PC.
The SoC initializes and test "its basic stuff" and passes control over to the GPU, which initializes the graphics engine and RAM allocation GPU/CPU (Because it makes sense the GPU has direct access to RAM for speed optimization) This is were the API also initializes and only selected people can view the source to develop drivers and interfaces.I think only some of the API sub code has been made open source, since its used across the world, in mobile devices - So letting the community play with code will only create more apps that run on this API "OpenGL ES". They most likely hide specific code in other binaries to protect intellectual property.
Then the CPU starts and loads the Operating System, and here the user has full choice over the what he want to load there, and it work as long as it has the drivers and is capable of running on ARM architecture.