I talked with Gedys Intraware (http://www.gedys-intraware.de/) about composite applications and benefits of composite applications. They've called out several points, one of them I want to describe here briefly since I haven't thought about it earlier.
They implemented an application in Notes 6.5 that you could conceptionally certainly call a composite application. The arrows 1 and 2 describe the flow on the screen. The implementation is not very easy though. I'm sure that hardcore Notes developers will claim it is not a big deal, but it is certainly not trivial.
The real issue however was to implement 3. There are links in the navigator that invoke actions on the Notes UI document in the right column. The problem was that this Notes UI doc never has the focus when the action is invoked in the left frame. So they had to do a native Windows dll call to switch the focus to the Notes UI document which obviously doesn't work unchanged on other platforms.
With comp apps in Notes 8x you can invoke an action on the Notes UI document and when it is executed the platform ensure the UI doc has the focus.