dtexec.exe allows us to pass values into a package using /SET and a property path. Why can't we do the same using the Execute Package Task?
I raised this on the forum here: http://forums.microsoft.com/MSDN/showpost.aspx?postid=2077788&siteid=1 and had a really good discussion that involved Cho Yeung.
The overriding argument for this seemed to be that a package that is intended to be called from another package can become a reusable "thing" without having to know anything about the package that is calling it. That is NOT the case when using Parent Package Configurations.
I urge you to read the discussion at the link above and also this similar submission: https://connect.microsoft.com/SQLServer/feedback/ViewFeedback.aspx?FeedbackID=294904