Well, installing recommended drivers on Ubuntu installs the Intel or AMD microcode, which is definitely proprietary, so I don't think that's a particularly reasonable argument.
The NVIDIA driver is the driver that yields the best performance and experience; AFAICT the only reason it's not shipped by default is because the kernel is then considered "impure" or something (because source isn't available to debug issues in the NVIDIA driver), hence bugs can't be filed upstream for any issues in the kernel if the NVIDIA driver is installed.
However, none of this is really the concern of the Chrome team. Their responsibility is to give the user the best experience, which they have defined as security, stability, and performance in that order. Since the nouveau driver is causing stability bugs, it's perfectly reasonable for them to fallback to software rendering and take the performance hit in order to preserve stability.
