Ryan Matthew Pierson recently blogged about the benefits of using both Windows and OS X. While I agree with many of his conclusions, there is another, very real benefit for cross-platform application developers to use multiple platforms and that's creating a better user experience.
Your users want a great user experience. While it's possible to create a user experience that is better than the one provided via native controls, that's not usually the case. Just using native controls is not enough; you have to think about the expected user experience based on the OS which users are running. Simple things like the spacing and placement of controls can make the difference between your app feeling intuitive and easy versus confusing and difficult.
Real Studio provides several features that make life easier in this regard, not just for OS X and Windows but for Linux as well. For example, you can create constants with platform-specific values for use in your user experience. Your File -> Quit menu item can easily be made to use "Exit" on Windows. Using constants, you can swap the locations of your OK and Cancel buttons in dialogs since Windows/Linux place them in the opposite order of OS X. And since some controls are taller on Linux, you can use constants to control the height of controls as well. This makes it easy to provide the native and thus most intuitive user experience for your end user. Yes, this takes more time but it will also help your application be more successful.
The biggest benefit of using all the major operating systems for cross-platform developers is to know the user experience your end users will be expecting so that you can provide it and be more successful.