Servlet 3.0 introduced the feature to serve static files from META-INF/resources directory. The specification only demands support for this if the META-INF/resources is inside of a jar file in the /WEB-INF/lib directory. Tomcat passes this requirement because it serves files from jar but not from WEB-INF/classes. It seems Glassfish 3.0.1 however supports both type of locations. It would be nice if Tomcat would support it too.
Application server specific extensions to the Servlet specification are a really bad idea since they harm portability - the reason why we have a specification in the first place. On that basis I almost closed this one as WONTFIX. However, I do see a valid use case and one that I suspect will lead to a change in the spec. Some environments, particularly IDEs, unpack all their JARs into the WEB-INF/classes directory. This would break static resources in JARs. Enabling this option works around this issue. Since this is a proprietary extension, it is disabled by default and controlled by an attribute on the StandardContext but is not part of the Context interface.