22s are also relatively quiet so they wouldn't attract a lot of attention in an urban environment.I voted important, but don't go nuts.
I was concerned about ammunition availability, reliability, parts availability, and redundancy. That got me to 5.56 ARs, 7.62x39 AKs, 9mm Glocks, 12 gauge 870s, and a variety of .22s. But I've tried to keep my whole collection restricted to commonly available calibers, and I've tried to stock an "adequate" supply of ammo for each piece.
Definitely want to give a shout out to the .22. I'm in an urban / suburban kind of environment. In difficult circumstances, I figure most of the critters I could poach around here would be best shot with a .22 as opposed to something bigger.