I totally agree that a 22 isn't a preferred SD round, but, if its what ya got, its better then the BG's knife. now if he has anything higher in cal then what you have, well you really better hope he isn't a good shot.
personally i would much rather carry a 22 then nothing, but if i had the choice i would carry a 38 or 9mm, some very basic facts are, even a 22 is loud enough to call attention to the fact that a gun has been fired, most times when you will need it outside of your home will be in a populated environment, like a city where you have come from some show or dinner, shopping whatever, point being, a gun shot much less several will draw attention, and police. and a 22 pistol is plenty loud enough to do that.
and lets face it, any situation where you needed a gun is going to go down in very few ways, surprised and unable to do anything but grapple with your attacker hopefully drawing your gun and shooting anywhere they are. or you will see it coming and be able to draw your gun and aim. even in one of the shootings that seems to happen all to often anymore, unless your near the shooter you will have time to take cover and draw, and in most of those cases the shooters were wearing body armor of some kind, your 9mm or 38 in my case wont do any more good then a 22 in that situation so your best bet is to still get away.
they make some really nasty 22 ammo these days, at gander mountain of all places i saw pre segmented hollow points will it stop the BG on first or even 3rd shot, probably not, but unless you hit a instantly lethal area with something of higher cal, they wont either, shot is shot, anyone that can shrug off being shot, isn't likely to care about cal.
if your normal but still going to commit a crime being shot or even shot at, will usually be enough to cause you to run, if you can take a hit, your not normal and nothing short of a fatal shot will get the job done.
while i would want more, if its all i had, i would be very happy to have it, cause it has more reach then my fists and more ammo then throwing my knife