You can use C# everywhere, but in practice it just seems like a huge effort to avoid learning a new language to me.

I have nothing against the language (writing C# is half of my day job), but all of these bridges are second-class citizens and built and maintained at the mercy of third parties just to interface with your target platform. Apple will never release updates for MonoMac or MonoTouch. Google isn't going to release C# Android SDKs or documentation. Mono for Linux always trails the Windows version and has significant community resistance, and Microsoft isn't going to integrate and maintain these bridges.

You're also always losing something in the translation too, be it idiomatic style of the target API or C#, or being totally cut off from new native features, or severely restricting what community support or documentation is available to you. Any way you cut it, it's a second class experience. Worse, some of the bridges aren't free. You get to pay $400 for the privilege of not learning a little bit of Java or ObjC to get MonoTouch for Android or iOS.

C# is good, but it's not so good that I refuse to learn other tools.