Basing a system's internal object-oriented design upon essential use cases has several advantages when a user interface is also being designed using essential use cases, typically via Usage-Centered Design. Duplication of work can be avoided: the essential use cases developed for the user interface design can be reused directly by the software designers, without having to be recast into other, more detailed forms of use case. This allows user interface design and software design to proceed in parallel, both working from the same essential use cases.
This is in contrast to the serial approach taken in many formal processes (RUP in particular [16]) where user interface design is performed early, in the inception phase, and that user interface design is then input into a later software design phase. Followed strictly, the serial approach delays software design until the user interface design is complete: for large, user interface intensive projects this can add several months to the time frame of the whole project.
Parallel design is particularly useful for web development, as the front-end interface can be designed separately from the back-end server software: each could be outsourced to different vendors with suitable expertise. Such parallel development does require some kind of iterative project management to coordinate changes and elaborations to the use cases as each design proceeds.