Yeah I think it comes down to what degree of interactivity the prototypes from tiggr will have and potential future of outputting not only static images or maybe code starting points for development.
But that aside, it goes back to the other thread question on things like "conditional logic" and if such logic would need to know about things like "required fields" etc. If done via any of the down and dirty and notably easy ways, the logic would never be able to dynamically interact with fields that say needed to be filled in before navigating to the next prototyped screen...
So true interaction behavior could not be entirely modeled, especially if you are trying to model advanced user notifications around form or field validations...