I have literally never even heard of my grid going down for 24hrs and I'm not talking constraint to my lifetime. To think it happens every 2 years is outstanding.
You also do not get any additional benefits. The cost is being able to run off grid doesn't get to any additional benefits like being sell back to the grid. Obviously you need to pay for batteries and stuff beforehand and the logical reason for buying batteries stands. But to upgrade your system to an event that has literally never happened before seems silly.
Spain just royally fucked up on a grid level and not something that is likely to occur again. They need to spend more on their grid which they are.