I would argue that while a .22 makes a good offensive weapon, it is a poor choice for a defensive weapon.
Here is my reasoning:
-If you are in an offensive position of an attack, you are already ahead of the game. You most likely have the element of surprise, and more importantly--time to prepare. As the aggressor, you have time to prepare your mission. If your mission is to kill Joe, you have all the time in the world to prepare for it. You choose the place, time, and method. You can sneak up on Joe and put a bullet directly into the back of his head. For this reason, I would argue that a .22 would be ok. It's not ideal, but you can prepare around it. You know that you only have one chance to pull this off, and all that is available to you is a .22 revolver, so you ensure that the fight is in your favor.
-This contrasts sharply to the defender. The defender has to react in the time allowed by the aggressor, which is obviously a bad position to be in. You basically subconsciously exploit the weaknesses of your attacker and his plan in an infinitely small amount of time. If you are not able to do so, or your plan does not work, you die. This is why it's probably not a bad idea to stack the odds in your favor. You can not prepare for these situations other than basic, all-purpose, general plans. If you are Joe, and you are smart, you get a CHL. You then buy a reliable self defense gun. Ideally, you want your self defense gun to give you odds over the aggressor. You won't have time to react, you will only be able to react in a time/manner that exploits the aggressor's weakness or the weakness in his plan. It would make no sense if you are Joe to prepare by bringing a .22 revolver to counter the aggressor's attack with a .22 revolver. In other words, where the offensive actor can plan around his choice in weapon, the defender can only react to the aggressor.
-So, why be reactive when every single one of us has the opportunity to be proactive?
I'm not sure if this makes sense to any of you.... but there it is.