This has been much discussed in the forums.
The V2 does indeed have batteries which will last for three or four hours.
The issue that arises is that occasionally after SmartThings pushes out a firmware update, the devices at a customer’s home will stop working and the recommended solution is to powercycle the hub.
That’s fine if you yourself are physically in the same building and are capable of doing that. (I myself am quadriparetic with limited hand function, and cannot push buttons.)
But maybe you’re traveling. Or maybe the hub is at a vacation home or a rental property hundreds of miles away. So how can you force a reboot? As you mentioned, if the hub is on batteries it will just go to battery status and that will not force the reboot.
There is a reboot option in the IDE, but if the hub is not responsive that won’t do you any good.
So quite a few community members have plugged their hub into a networkable switch which is usable even when SmartThings is unavailable. Commonly a WeMo switch, but the switch discussed up thread also works and is less expensive.
To use this method, you have to take the batteries out of the V2 hub.
So then some people go one step further and add a UPS as well. But still one that can be powercycled remotely even if SmartThings is down.
So the use case here is not about providing alternative power to the hub. Rather it is about providing a method to power cycle the hub when it is nonresponsive.
As I mentioned, much discussion in the forums. The following is a typical thread.