An armed citizenry is important. But as you noted, it is of limited value. For example, armed citizens could not have prevented the Boston Marathon bombing.
No, sadly it couldn’t. And that would become a common tactic to be used if gatherings of crowds that were armed. If I were a terrorist and I wanted to make the biggest impact, and even though I was committed to die, I certainly would not walk into a hail of bullets from armed citizens.
Being armed is supposed to make you safe and FEEL safe as well. If you being killed by tactics and techniques that negate you being armed, you become demoralized in spite of being armed.