I disagree: The argument seems to have escalated verbally for a while. The Marines were obviously outnumbered and in danger. If they had been carrying, the man with the knife should never have been allowed to get anywhere close, and probably would not even have tried.
Yeah, but that takes discipline and being alert to the danger. In this case, five guys spilling out of a car after a verbal exchange should be an indication to forget being macho and draw your weapon.
On the other hand, strolling down a street and having someone stab you with no warning, or cold cock you without warning (as is case in most of these incidents) is a whole nuther issue. Even Zimmerman wasn't able to prevent the initial attack. If Trayvon had a knife, no doubt he could just as easily have fatally stabbed Zimmerman.
The key is the ability to maintain a safe distance and drawing your weapon sooner rather than later improves your odds of being able to do so.
There are a number of training videos that go into detail as to what the minimum distance should be and just how few seconds you have to react. The distance is quite long and the time to react, short.