Julian,
I don't think anyone wants to see another Sept, 11th period, howsoever caused. Since these awful events I venture to suggest that passengers are much more likely now to take matters into their own hands should there be the slightest possibility of such a happening ever again. Maybe the terrorists of the future won't need anything sharp or pointy to instigate their plans to take over the aircraft, rendering conventional security impotent, and in that situation wouldn't you feel safer knowing that your fellow passengers might have a fighting chance of staving off disaster by attacking the hijacker(s) with whatever they had to hand, just stuff people normally carry around with them, yes including penknives?
Would you, as a passenger, in the light of the WTC attack, be content to sit and freeze if someone tried to take over your aircraft, or would you fight like hell to save your life? Mightn't a penknife come in handy in that situation? There's a lot more passengers than hijackers, after all.
Otherwise, security guards might as well truss us all up in strait-jackets before boarding and have done with it!