I think there are two ways to use a .22 caliber for self defense. We use one way often.

A .22 lr revolver (that we have) is shot very often by both my wife and myself to keep in practice and just have fun. We both shoot our .44's also but not as much as the .22. We believe that shooting the .22 keeps us better shooters with our real self defense revolvers. (My wife seems to outshoot me fairly often with the .22. She never let's me forget either.)

In an emergency, hopefully better armed, a .22 would be better than nothing and if I had to I would use it. I have killed hogs and cattle with a .22 to the brain so I know it can be deadly, but only with a careful shot.