Often, DWR is not used just by itself. Perhaps, you have an on-going project where decision has been made to use a framework such as JSF or Spring for building the whole solution. In these cases, AJAX and DWR are just a part of the overall picture, and so DWR needs to integrate with other projects nicely. And that it does.
DWR integrates with the following:
JSF (http://java.sun.com/javaee/javaserverfaces/), Java Server Faces for building user interfaces
Spring (http://www.springframework.org/), a complete Java/JEE application framework
WebWork (http://www.opensymphony.com/webwork/), a web application development framework
Hibernate (http://www.hibernate.org/), an object/relational persistence and query service
Integration to a portal such as Apache Jetspeed using JSR 168 portlets is also doable; for example, see the article on IBM developerWorks (http://www.ibm.com/developerworks/java/library/j-ajaxportlet/)
And many others, whose integrations are provided by DWR...