When actually confronted by a bad guy who wants to hurt you, there is no caliber that is not marginal. I've seen a few PCP enhanced folks against whom I might have wanted a 155 mm howitzer.
But for walking through the neighborhood at midnight, walking the dog at zero dark hundred, or as a backup tucked away in some unmentionable part of one's anatomy, a .380 will still make you feel all warm and fuzzy. And if you practice with the gun and the ammo you will be carrying, a .380 is far from ineffective. A .380 to the head will end any argument that I have ever witnessed. The purpose of a .380 is to allow you to go home at the end of the argument. You will not take down a terrorist at 1000 yards with one. You will not shoot holes in an escaping car with one. And those limitations are probably good.