I would say if a piece of functionality is reasonably distinct such that it can be separated from other concerns and this separation generally makes the code better maintainable than such separation is justified. Separation is required if there is a need to plugin alternative implementations (i.e. the ResolverPlugin)

ServiceManagerPlugin
http://fisheye.jboss.org/browse/JBossOSGi/projects/runtime/framework/trunk/src/main/java/org/jboss/osgi/framework/plugins/ServiceManagerPlugin.java?r=root:

Is an example of a well defined and reasonably well documented interface that the rest of the system can interact with.

The separation of ControllerContext and OSGi Service handling is actually something I'm not so happy with. AFAICS, ControllerContext handling should be an implementation detail of service handling and should only be done within ServiceManagerPluginImpl. This is currently not easily possible because the methods in ControllerContextPlugin are called from many different places outside the ServiceManagerPlugin. This should perhaps be cosolidated unless there is a good reason to separate ControllerContext handling from service handling.

https://jira.jboss.org/jira/browse/JBOSGI-278