PC sales are declining, in the tens of millions. Smartphone sales are still skyrocketing, in the hundreds of millions (and into the billions very soon). There's a reason Microsoft are sinking so much money into trying to be relevant on mobile.
It's not a lot of use without any scripting support. And no, I'm not privy to the details of any financial arrangements between Unity Technologies and XamarinUnity uses Mono to support scripting, which includes UnityScript, Boo or C#, Unity itself (runtime) is not written in C#, it's native and IIRC it's written in C++. How is that 'based on Mono'? By the way, do you know if the Mono devs get even a penny out of Unity's use of it as a scripting engine?
Anything less than majority is irrelevant?By it becoming the dominant development platform on mobile.
Actually, you can write for Android in Qt (although it is not complete yet, there are a number of KDE and Qt applications with android ports), and with Qt 5 android support will be increased considerably. An iOS port of Qt is also in the works by the community, although I don't know the current status.
Microsoft don't care if you write applications in C#/NET or in C++, as long as you do it on Windows desktops/laptops and Windows phones (and soon Surface). C#/NET is Microsoft's answer to Java in the enterprise, here we are talking consumer mobile. Meanwhile Visual Studio 2012 is branded with the 'going native' slogan with a major push in improving their C++ solution. WP8 will support native applications written in C++, which will run on top of the native WinRT runtime. Not even on current Windows mobile platforms are you relegated to C#/NET.
It's even less useable without the Unity runtime.
If you are to earn the epitet: 'the platform that matters on mobile', yes.