There's a commercial incentive to build electric storage, if the price is right; you can buy when prices are low and sell when prices are high. Depending on the market regulations, prices are likely to get high during an outage, so there's your motivation for construction. Of course, siting is important too; if the big backup battery is right next to the big power plant, that helps for power plant outages, but doesn't help for wiring faults. And a battery facility that's not sized to power the grid island it finds itself in likely can't provide any power, unless the grid island has sufficient load shedding mechanisms, I'd guess.
