But unless your are a real FOSS zealot (say > 500milli Stallmans) you are at the mercy of the bios, the disk controller firmware and the CPU microcode on any machine
In reality being able to run whatever user programs you want in whatever combination you want without artificial is what FOSS is all about.
The "user programs" you want to run are determined by the operating system. And the operating system is determined by blobs, controllers, BIOS and similar gunk.
If for example the Pi blob is such that Linux kernel 3.7 can't run then you are SOL. Or if on a device like this their blob only works with Android 2.3 then you can't run a different version. Or maybe you can't run one of the BSDs.
The video/GPU is especially relevant in the Pi case because it controls what the CPU can do.
Yes that's true - I was thinking of people who complain they don't have details for some deep detail of the GPU shader cores to write an opensource driver for their NVidia card.
In reality being able to run whatever user programs you want in whatever combination you want without artificial is what FOSS is all about.