Put walls between people, and you'll need to hire more people to coordinate them. Dirt basic common sense, yet one that most organizations have a hard time with. That's a cultural issue. Everyone's allergic to working on tables.

Someone once wrote a comparison between DB2's architecture and Ingres. Both are the exact same kind of application built on similar kinds of machines at around the same time. Yet Ingres was built as 4 separate process. The Ingres team was organized in 4 separate groups.