In a RAD environment, like Delphi, I see very little value in modeling the forms and dialogs in Rose. However, we use the Observer pattern, this I think is one of the things that should go in to design. When picking up the system for maintenance, I surely would appreciate getting this information up front before looking at the source. I think that the main purpose of a model is to show the relationships between objects and classes. Therefore, the most important things to show are the associations, dependencies, class public interfaces, and how the objects use each other's public interfaces when they interact.
Diagrams are a design and communications tool.
The people who need to code the design, have enough information to produce that code. The people who need to maintain the system can use your diagrams as a roadmap into the code, and as a vehicle to understand what the code is doing. Unfortunately, all three of these points are highly subjective. You can't really tell if 2 and 3 have been met until its too late. And it's way too easy to wave your hands around and pretend that you have met 1.