It is also a set of re-usable components assembled into a framework. You can choose to work with just the Servlet, you can choose to work with just the components or you can choose to work with both. That is what we have done as well, however we have done it in such a way that it is very pluggable. Mix and match and use what you want to use. Ignore the rest.
Turbine provides a framework for developing web applications - specifically the front end.
Turbine provides facilities for plugging in other Services - and thus JMS could be plugged in as a service to that. Even if it isn't plugged in through Turbine's Services facilities, you can still put your JMS related methods into your code. Turbine doesn't prevent you from calling other Java code. Turbine provides access to JNDI via a Service facility. Again, nothing preventing you from using XML within Turbine or even sending XML out of Turbine.