In general, there is no good reason to install a distro with proprietary graphics drivers by default.
You have to look at the complete picture in linux, and use the system to it's fullest capabilities.
It's for instance much more useful to install a distro with the KMS-drivers (which is always an option, mostly it's default) for your base system, and then to install SteamOS or Sabayon or Manjaro with Steam and proprietary drivers in a container on your base system with a PCI passthrough, or if you don't have VT-d/AMD-Vi capable hardware, you can just install it in an lxc. Linux has all these capabilities, there are so many elegant and reliable solutions.
I for instance think that users that don't want to clown around with proprietary graphics drivers, should just run a container (either kvm full virtualization with a PCI passthrough, or an lxc if their system is not IOMMU capable) with SteamOS. You can be damn' sure that Valve is going to make SteamOS work with proprietary graphics drivers, and for the host system, you don't really need them anyway, especially if you're on Intel or AMD graphics and use a bleeding edge system. Just take the scenic route through the linux landscape, don't solve problems that can be easily avoided, stay relax, don't fight it lolz