[edwinnathaniel]

Not every app has to be beautiful and super-usable though (most business/productivity apps).

If an app can achieve 80% of the usability cross-platform without doing much tweak, I'd say that's a good investment from the publisher perspective.

[migueldeicaza]

Agreed, not every app has to be done in that way, and for that there are plenty of alternative solutions.

From HTML to Java, to Gtk# and a dozen more.

This is a set of tools for developers that need to take advantage of native APIs, either for the UI, accessing the advanced audio features in OSX, accessing Darwin features, integrating with StoreKit, iCloud, Apple's OpenGL extensions, SceneKit, Bluetooth and those sorts of things from C#.