>What's the point?

You've completely missed it, apparently. Why do you need 100% offset? The grid is your battery, anyway, so you'll still have to pay a connection fee.

I've got 66% 'offset' from my system, and I'm glad of that. It's diminishing returns at larger sizes, anyway. It'll pay back in 8 years, and give me a return for the following 13+ years.

What was the payback on your last car purchase?

>But what if your region occasionally has power outages? Sometimes a back-up system is not about ROI, sometimes its about having electricity to run the gas heater controller during a cold spell.

Backup power requires batteries and a suitable island-mode / EPS enabled inverter, which is additional to a normal install.

I'm not saying you're wrong, just that you need to think of these things up front.