The reason it is not present in 2005, i think, could be because tools like ERwin are much better at both forward & reverse engineering. From what I know we can add or-and create tables, design relationships and do basically everything which can be done in table design. Another benefit is that it can be useful for documentation purposes.
Are there any other benefits of the Digrams in SQL server EM?
What I hear I forget, what I see I remember, what I do I understand