A Powerwall doesn't store enough energy to be worth much in a real emergency. It's going to be nice to keep your refrigerator running during a short power outage, but it won't provide days or weeks of power the way an inexpensive generator can (for example http://www.homedepot.com/p/Generac-11-000-Watt-Air-Cooled-Au... ).
Sure, you have to either trust that nat gas is available or deal with fuel, but a propane tank isn't obscenely expensive considering that it bumps your backup from hours to days/weeks.
I don't understand your argument. A Powerwall, if it can get you through the trough, means in an emergency you have the same amount of power you'd have in a non-emergency.
A few years ago in Vancouver a storm caused quite a number of major but localized power outages, some of which took a few days to fix. During that month or so there were a few weeks where the sun was blocked for almost the entire day - not even like overcastt.
That being said, I'd you're in a more inland area, with more sun and less clouds it might make sense.
That might work for food in the freezer, as long as you don't live in an area with wildlife like bears and raccoons. It seems like a huge market for this would be off-grid people, who are going to live in the country where such animals will live. It seems like raccoons are in most American cities anyway.
You are still going to have a problem with refrigerated food - lots of vegetables spoil when frozen and frozen milk isn't much fun. Plus the house is going to be consuming lots of power from lighting and TVs as days go by.
I have family that live in very rural areas. Most of them have diesel generators or tractor PTO generators to cope with power outages that last many days. Such a situation is certainly is something that is survivable without a generator, but not very pleasant. Going through the experience once, many people realize that generators aren't that expensive and buy one to be prepared next time.
You might also wonder about running out of diesel fuel, but most farmers have 500 gallon tanks of (untaxed) diesel for equipment. Hundreds of gallons of diesel lasts a long time.
And if you have something like a dairy farm, a generator is a necessity for such a situation with a milking machine.
Storm-damage-related power outages are the killer, particularly if you rely on air conditioning. The last major storm in my area that impacted me knocked out power anywhere from a few days to several weeks, depending on the area.
It is decidedly unpleasant to be in 70+ degree heat with 80% humidity at night and hotter during the day.
Sure, you have to either trust that nat gas is available or deal with fuel, but a propane tank isn't obscenely expensive considering that it bumps your backup from hours to days/weeks.