You may pour as much scorn as you like, but this is the reason given to security personnel, during their training, to be particularly vigilant over these tiny items.
We know for a fact, because this is well documented, that flight attendants were killed before the hijackers gained control.
No disputing that crew and passenger(s) were stabbed and some killed during the takeover. However there is no evidence of the weaponry or exact details of the injuries such as you state. And while it may be er.. useful to embellish for the purposes of training security staff, it should not be stated as fact. I leave it to individuals to decide the appropriateness of such training methods.