For that matter why do we need Flight Attendants surely airlines could have drink dispensing machines etc.
I remember seeing a really crap movie on this very topic, a pissed of ex-employee hacks into the aircraft in flight and takes over. Hell of a lot easier then sending a suicide bomber to do the job, plus you get to use him again
While it is true that most accidents are in someway human error, it is also true that more accidents are prevented by flight crew then caused.