I think we are viewing things from a different perspective.
First of all, I don't see a point in using linux and depending on proprietary drivers for my graphics card. That goes against what open source is all about. And the AMD open source driver is on par with Catalyst according to the absolute latest git build and the Valve patches (http://www.linuxsystems.it/2014/05/radeonsi-awesome-beats-catalyst/). And it will only get better now that Valve is pushing SteamOS.
Secondly, what type of distro are you using? Because with a bleeding edge distro, where kernel version are behind the latest stable versions by a week or two (I'm on 3.14.3 on Arch linux, latest stable kernel is 3.15 released on Friday), the NVIDIA proprietary drivers often don't compile against the latest kernel, and you have to wait for the distro maintainers to release a fixed version of the driver. You don't have this issue with AMD (both Catalyst and open source). I am using bleeding edge distros as an argument because kernel versions are usually made up of 50% drivers. Newer hardware will be better supported on newer kernels, which get released typically every month, on a Sunday.
And lastly, NVIDIA has treated the open source community with the outmost contempt. They lied about releasing the kernel hooks for their drivers. I heard that some guys even got crowdfounded to rent the scientific equipment to reverse engineer the NVIDIA cards (I don't have a link at hand) because they wouldn't release the specs. And NVIDIA keeps mocking the Nouveau team, when the Maxwell GPUs were launched they sent them two 750 Ti's and said that is all they can do to help the open source driver development. NVIDIA is also trying to get into the Android market with their chips, and they offer no support for linux development (Linus Torvalds specifically states that NVIDIA is the worst hardware company to work with). These are just the things that came to mind off the top of my head, I am sure there are other instances where NVIDIA has demonstrated how against open source they are.
EDIT: just remembered this, so funny: NVIDIA has a contract with Red Hat. Red Hat sponsors Fedora. The Catalyst drivers got orphaned (removed) from the Fedora repositories. And like I've said, Catalyst compiles without a problem with new kernels, you don't have to fiddle around like you do with the NVIDIA drivers, yet they are orphaned. These are the kind of tactics that NVIDIA uses in the open source world. Basically, Microsoft level tactics.
EDIT2: I am looking at the Nouveau home page, it seems that NVIDIA is beginning to offer a hint of support for the Tegra K1. Too little, too late.
EDIT3: I keep coming back to this, sorry for the huge wall of text. A PCI passthrough to a Windows VM, so you can play Windows games with almost no performance loss, is extremely difficult to realize with Nouveau, impossible with proprietary drivers (both AMD's and NVIDIA's). It's very easy with the open source AMD drivers, there is a tutorial made by Zoltan in the linux section of the forums, and basically every distro forum has tons of post about how to do this.