First off, I think Tivo shipping was a very important milestone in the domination of Linux. Before that there were not a lot of Linux based home appliances, they demonstrated that Linux could work in large scale deployments to people's houses (and for years afterwords I still had arguments with people claiming that Linux could not be used in that type of environment)
Second, the Pi allows you to run with everything running in the Linux space being open source, this is FAR better than most computers with reasonable graphics do.
If you are so worried about how the binary blob in the GPU can access the memory that the Linux kernel uses, then you had better not have any PCI cards (especially not any PCI graphics cards) in your desktop PC, because the binary blob firmware running on any PCI attached device can access any memory in the system (unless you are using IOMMU and the device driver has been written to limit the hardware)
Would it be better if the bootloader/GPU firmware was open? of course it would be.
As for the reaction from the people at the Pi foundation to 'polite' criticism, given the abuse that they have suffered, I think it's a matter of the well being poisoned, after being abused by so many obnoxious people, their patience is long gone and they don't react well to anyone. This doesn't mean that they don't care about openess, it's just part of human nature.
> If they were to acknowledge how closed their platform is in comparison to a standard PC that would be one thing
I don't believe that their system is closed compared to a standard PC, so I can easily see how they don't either.