While shot placement is important, you still need to stop somebody. The best stats I've ever seen on a .22 LR taken from real experiences was a 1 in 4 chance of a one shot stop in the core of the body. Compare that at high 80% - Low 90% for the modern 9mm rounds and high 90 percentile range for the .40 and .45. I'll try to come up with where I found those numbers, my memory is a bit vague on me. I would be good with a round as small as the .380, some of the ballistics on the modern hollow-points seem to be outding anything I've ever seen out of a .38, including the +P rounds (assuming that the .38 is shot out of a 2" J-frame). But, something is better than nothing. Right now my wife is still intimidated by anything much larger than a .22, but we're working on that.
As a side note, the military is limited to the 9mm ball (full metal jacket) round by the Geneva conventions, and carry hollowpoints whenever we can (US soil, to include sate land overseas such as embassies). The complaints you see from downrange are using the ball round, which I would never use for self defense unless I had nothing else to shoot.
Cheers!