Any time you can get more energy out of the barrel up to a certain point, it will benefit you in a defensive situation. The .22 magnum is a nice hunting round for small game and has been used successfully as a defensive round. Keep in mind that any rimfire will be less reliable than any centerfire. I've seen many stories of the Ruger Single-Six .22 mag being used against bad guys, even terrorists. Like the smaller LR, it enters the body, but doesn't have the energy to exit most of the time. That said, the .22s lethality is often greater than its initial stopping power.
If I were alone at night somewhere, I'd feel comfortable with a Ruger Single-Six .22 loaded with the magnum ammo.