A small gun is better than no gun, if the user trains with it and knows its limitations. Producing a quick draw, and several shots on target is definitely going to initiate the FIBS/FIBSA factor in most SD situations and do the job with a low caliber like. 22, 25, 22 WMR and .32, etc.. Some situations where an imminent deadly threat needs to be stopped very very quickly, then you'll wish you'd had a bigger service caliber with quality hollow point ammunition, one that you've trained with often. These will provide better threat stopping effects and quicker, for the self defender.
I've pocket carried my .25 ACP (Beretta 950) on very rare occasions when the weathers hot and im at a summer event of some type where I just don't want to print or give any hint whatsoever I'm carrying.
I definitely can see certain bad scenarios where I would comply, evade, avoid (with the .25) to a higher degree than if armed with the usual weapon. I wouldn't launch a counter ambush unless I knew I could effectively stop the attacker(s). The .75 second, 1 second, and 2 second rules might not apply while I carry the .25 is what I'm saying, those rules no longer are effective. Very specific training criteria needs to be thought about carefully on when you would use the weapon in order to stop a threat.
In advanced training, there are certain cues and time margins to go on. All of these cues and time margins need change a lot when you have a dinky little gun and you decide to initiate your counter ambush... Or it could cost you bigtime.
I don't like small calibers for CCW, I believe they are more of a liability in some cases than higher service calibers.