- if you write a Gtk# application, go to to Mono website to download the GTK# installer on Windows
- if you write using SIMD intrinsics, even you'll be using Mono.Simd, .Net version, it is made just to compile, but is done in software
- if you want a C# REPL console, you will go back to Mono's gsharp console
- if you need a compiler/runtimes that compiles C#, and is embeddable in all forms and shapes, this is Mono
- at last, if you want to fix an issue, you can do it, because is free/opensource software
Those features are features I thought in 5 minutes.
Why do you consider that Microsoft should go and make Mono to be on feature parity with .Net. If it would ever happen this, the same folks as you will say: why Microsoft tries to add things in Mono, certainly they are hiding something.
And if you take Mono on a wider context, you can target Cocoa#, MonoTouch, Android's frameworks, OpenGL (which is not supported in .Net, but in an independent translation like Tao or OpenTK), SDL (via Tao), and so on.
At the end, did you worked with Mono and you find no practical way to implement a Linux only application using C#, when MonoDevelop is the C# IDE that is best supported in Mac OS X and Linux, and shows to you that all those things are possible?