From my experiance, Glass fish is ONLY used by people following the JEE tutorials from oracle (using netbeans too). It is not a competative-performant-scaleable JEE Application server.
You seem to have limited experience then. Glassfish is the reference implementation of Java EE standard and therefore it is used in JEE tutorials. BTW, IT IS used extensively in many enterprise application, including very demanding stuff like stock broking and trading (I have designed it for a large customer myself who serve more than million trades a day, so I can speak with some authority). This is a big news exactly for the same reason. There are many enterprise customers who paid money to get commercial support on Glassfish. Now those companies will either have to depend of the community for support or switch to other commercial options like WebLogic or WebSphere or JBoss EAP.