Jump Seaters are a safety issue.
On most aircraft the jumpseat blocks access to the cockpit.
So if you select a passenger at random and ask them to sit up front you will enhance safety by increasing the length of time it takes for an uninvited guest to interfere with the pilots.
Also, I've never had a jumpseater, whether airline staff or otherwise, who didn't enjoy it.
Its good
PR and its cheaper than modifying the doors.
BoarderDude,
I don't underestimate the threat, grew up with it. I do however feel that the door issue is a RE-action. The most important thing is that we are aware of any threats and have some form of plan if we ever encounter them. I know of some-one who actually had some-one rush the cockpit and go for the power/condition levers. They had a plan which worked and were aided by other pax. years before 9/11.