And all because
...Tesla need to build battery volume to drive down car production costs. The real (ie wholesale) price arbitrage on electricity over a few hours (less the 25-30% losses into and out of storage) is insufficient to pay the costs of home power storage system like this, but Tesla hope that ill-conceived solar subsidy programmes will mask the harsh realities.
As the article says, the system has been hawked around utilities and I suspect that they've run the numbers and they don't work except in niche situations *cough, cough, AC and all that*.
If you can launder the subsidies by using such a system, then you might have some small rationale for using it, but as I understand the rules both microgen and storage must switch off if the mains goes down, so even the theoretical value of UPS is not available.