Suppose the following scenarion: you have an UserDetailsService implementation declared in a config file loaded by a ContextLoaderListener. You declare an <http /> namespace element in the config file for the DispatcherServlet. When you try to launch this webapp this causes an exception thrown by AbstractRememberMeService claiming no UserDetailsService was found. SO somehow the namespace avoids to find the bean in a possibly available parent ApplicationContext.
A workaround to this behaviour is to explicitly define a <remember-me /> element and let user-service-ref explicitly point to the bean to be used.