Integration of different frameworks together is usually done because each framework is intended for a specific task. Hibernate for example is great for DB access, since it maps your DB tables to the object domain, so you don't have to go into the usual
JDBC hassle. It also hides DB implementation and vendor specific SQL from the developer. Spring is great for wiring multi-tier applications, and it integrates with different frameworks to make your development easier, and make you concentrate on your business logic. It supports and eases up AOP, and lots of other features. Velocity is great for templating, while tiles and site mesh are great for creating web views. We cannot have a framework that will give us all of these stuff just to not integrate different projects together. Even spring itself, having so many features, it's divided into modules that you can use separately, and some are even separate sub projects.
To sum it up, you'll always have to have several projects integrated together to make the most benefit.