I once put this exact argument to a network vendor's rep at a conference, after they'd introduced a really thin shim like this, with some fanfare. You know what his answer was, live on stage for all to hear?
"but it's open source"
If open sourcing proprietary code is the goal, as it has always been mine, then this sort of thing is very much a step backwards.
This also opens the door for any other kernel (like the BSD's) to speak to Nvidia's firmware without needing to do any reverse engineering. As well as gets Nvidia out of the way in terms of adopting new stacks like DRM-KMS. They are just no longer a blocker on that now since that's all handled by the now open source "shim" (which is still more than a shim)
Its a step backwards too, because the firmware is now signed and therefore unmodifiable, but that was a problem before the recent code release too.