It seems to me that there are some who just can't wait to demonstrate their superior skills in the use of handguns.
As somebody probably already have stated, there are three lines of defense, 1. prevent the hijacker from boarding, 2. prevent the hijacker from entering the cockpit and 3. prevent the highjacker from taking over the controls.
If the hijacker manages to board a flight, it will be very easy to prevent him/her or them from gaining access the cockpit.
It does not take a lot of time, effort or money to design, manufactur and install a cockpitdoor, that could only be breached from the outside if explosives were used.
So point no 2 and 3 should be seen as one.
But of course that does nothing for ones ego.