Matthew,
The roadmap diagram stuff is fairly recent and what I like to call a "sales feature".
It's only true purpose is to allow the sales people to say "Look, we do Roadmap diagrams as well", but it hasn't been thoroughly designed and tested.
It probably only works well for the use case: "demonstrate the feature to a potential client"
As with many of these exotic features in EA (testing, debugging, project management, instant chat, email,...) there are most often better tools on the market for such tasks.
EA is really good as a general purpose modelling tool, and to generate development artifacts (code, database definitions, XSD) and documentation, but outside of that scope I'm not sure it is the best tool for the job.
Geert