More people in the cockpit would have made no difference to September 11 as I understand it. The hijackers gained access to the cockpit by slicing the necks of a few FA's or threatening to blow up the aircraft with a (fake) bomb. In their situation at that time, I think I would have opened the cockpit door too. Don't you? Few people, if any, thought that that type of attack would ever happen. There were no plans for it and that's why it worked. I don't believe it would now (and events such as the shoe bomber back that up). Perceptions have changed. People would prefer to "go down fighting" than sit passively while being flown into the Capitol Building.
So, denying non-flight crew access to the cockpit will potentially stop the FedEx/BA type hammer-wielding incidents, but please, lets stop deluding ourselves that it would have any great effect upon non-flight crew seizing the aircraft. Screening is the answer to a point, however, we all have to accept that we live in a dangerous world and there is no way of achieving a 100% prevention rate in stopping these sorts of events happening.