In 4.5+ of the Flex SDK there's still no Sparks flex component that replaces the ViewStack. I know that there are several ways to mimic that behavior with states or includeInLayout properties. I also know that there's a couple people out there that have created their own sparks ViewStack.
My question is why is there no official replacement and what are best practices for the replacement of that functionality moving forward.
You are correct, there are several ViewStack implementations out there. One good one I found was Todd Anderson's here: http://custardbelly.com/blog/2009/09/02/viewstack-in-flex-4/
But moving forward the best practice now to get the functionality is to use Views and a View Navigator. Check out more information here: http://opensource.adobe.com/wiki/display/flexsdk/View+and+ViewNavigator