Full disclosure: I am the current Spring Integration project lead and have been a committer for over 5 years.
While it is true that Spring Integration promotes modularity and loose coupling within an application, it is also very well suited for integrating systems together without the need for a central ESB. I am personally aware of a number of very large enterprises that integrate all their business systems together using only Spring Integration, with no central bus server(s) to configure/administer.
Its POJO programming model makes it incredibly easy to customize/extend; if some transport/protocol is not supported out of the box, you can simply wrap it in a POJO and invoke it (or consider writing a more formal adapter and contribute it back to the framework!).
We are particularly excited that it forms the basis of the new and important Spring XD Project.
You may want to take a look at DZone's recent Guide to Enterprise Integration which talks about Spring Integration as well as the competing technologies.
