1) On Linux, every quirk someone gets with the Catalyst is seen as evidence for the bad quality of the drivers. If quirks appear with NVidia's blobs, it's seen as single and particular cases - the overall quality isn't questioned.
2) Assuming NVidias driver quality really was that much better on Linux, what's it good for? For everyday, non-3D usage, Intel's open-source driver outclasses both blobs. And for 3D-demanding stuff (like Games) it won't help if case such as Tomb Raider appears.
I've been gaming for around two decades and following AMD (or ATI) and Nvidia since I ATI rage 128 and Riva TNT (before that I wasn't aware of what hardware I got) and both companies sometimes just suck. Mostly I (try to) ignore the tendency of the Linux users to see everything either black or white (good/bad, savior/villain, you name it), but gaming goes beyond the Linux world's nose.
So right now, it's hard to tell whether Valve will go with NVidia or AMD. The only point I can see from here is, that AMD already delivers the hw for PS4 and the next XBox, so they may offer further hardware off the shelf for Valve. I think, everything else is just wild speculation and/or fanboyism. The gaming scene still is (in general) very childish and pubertal, but even they managed to get over this (mostly).