The following is just my 2 cents, for what that's worth. I don't pretend to speak for all of Adobe in this respect.
My approach to building composite applications is to think about each micro app (tile) as a separate entity rather than to think of the application (shell application) as a whole.
For example, If I have an application that consists of a list of customers, their profiles and their current open cases; I would build it as three separate apps. Yes those apps need to communicate. But, if you do the communication through the Mosaic framework, then they can be independent of each other and thus they are more apt to be reusable (for example the list of customers may be used for reporting in another application).
With regard to using Parsley to create tiles - My opinion is that it is a bit redundant, especially in light of the upcoming release of Mosaic-next - the Composite Application Framework (CAF). When I look at what Parsley gives me, I think of messaging, injection and object abstraction. I can use the CAF context binding, Gravity and service objects to get the same thing.
