I actually have dabbled with it and I think Xamarin has done some very impressive work there. But most of the work in the typical app is in the UI and you still have to essentially rewrite that for every platform with Mono. For some apps it could still be a net win.

Native has the same advantages on the desktop but that hasn't stopped it from being pushed out of mainstream app development and into a few select niches.

Unless, of course, web apps are the native apps (e.g. Boot to Gecko).

I find it really ironic that - thanks to Mono - C#, a Michaelsoft language, seems to be the only true multiplatform lanugage these days.