That might apply when asked why they brought weapons, though...
I don't think so, for two reasons. First, possession of a weapon doesn't require any justification under self defense, unless the weapon was used to project force.
But also, if a defendant admits to bringing a weapon because he expected trouble, that plays directly into the charge of conspiracy.
A better reason for having a weapon on you is that it is your normal practice, and while it might be used in a self defense situation, one hopes that situation never occurs. "Habit."
I do agree, for those who are charged with using force or threatening force (for example by brandishing), self defense has play.