Traditional 2x4 and 2x2 Duplo bricks can be used to bulk out models and provide a structural support for landscapes, but have there been any other uses for Duplo bricks of all shapes and sizes in System scale MOC's?
Generally, I would say that it's only the big, strangely shaped ones that have decent uses.
Obviously you would very rarely use any of the small ones, and system is much better at this. (Only exception I can think of is a piece shortage (heaven forbid!))
I personally have never come across a MOC using DUPLO pieces in large scale, and I doubt you'll find a proper LEGO set using them either.
I would always recommend using system bricks if you can (you probably already know this).
DUPLO bricks do allow you to be lazy though!