How does Mir have better Nvidia support than Wayland?
They are not that well documented, but they work ok, because if not, the manufacturer does not sell the device.
Mir inherits Android graphic system on purpose.
In wayland you need Open drivers, like Nouveau which are very slow as the power management is not complete, or they are not sure enough to activate it(it could toast thousands of cards): http://nouveau.freedesktop.org/wiki/FeatureMatrix/
The life of those developers is hard, all day fixing bugs, and new bugs appear like it is never going to finish(you don't see the light at the end of the tunnel).
Or is all of this just because the Wayland and MIR developers only want to use open-source drivers?
I didn't know that, makes sense. That said, Mesa appears to provide EGL on top of GLX, which probably isn't as efficient as a driver that directly provides an EGL interface, but likely 'good enough' for basic desktop rendering. So I still don't really understand why I would need Nouveau to use Wayland or Mir.
For me, using Nouveau would be a definite showstopper for whatever new display server becomes the default in Linux distro's. Not to talk down the efforts of the Nouveau developers, but I only have bad experiences with the driver, ranging from crashes, screen corruption, unbootable installations, terrible performance and loss of indispensible features such as proper multi-monitor support and hardware video decoding. One of the first steps in installing a new Linux system on Nvidia hardware is usually to remove, purge and blacklist Nouveau, and install the closed driver, because that just works (tm). I'd even use software rendering over llvmpipe before I'd consider Nouveau.
If the future of the Linux desktop on Nvidia hardware requires a reverse-engineered GPU driver that lacks all the features provided by the closed-source driver, we can postpone the 'year of the Linux desktop' for at least another decade.