The Sonos does need mains, but that just means you need a free mains socket in the room you movie it to.
We have 4 Play 1's around the house (we're on 3 floors), and will just unplug one and plug it in where we want it if we need to change the configuration. They pick up the wireless signal in under a minute, then you're good to go again.
Since we've a (waterproof) socket in the garden we can take one outside if we want - all they need is mains and wi-fi.
I even took all 4 of them, and an old wireless router, to a hall we hired for my god-daughters birthday party. Plugged the router in, it created the wi-fi network for the Sonnos and my tablet (it had the same SSID & IP Range as my home network), and in a couple of minutes I had speakers in the 4 corners all in synch - easy