The Roam in standby/sleep mode will deplete it’s battery anyway, as the WiFi adapter (in low power mode) will be listening out for a WOL packet, updates and ip-lease renewal etc.
When the S2 App opens, it wakes the Roam from sleep for a couple of minutes only, if not used. Even if that were to happen say a half-dozen times per day I don’t think it would make that much of a difference to it’s battery-depletion time - you might perhaps save 30 minutes per day, compared to leaving it in sleep-mode all the time.
A real battery saving would probably only come about by switching ‘off’ the Roam completely (which is the current 5+ second power-button press until the power-off descending jingle is heard).
I don’t think you would save that much power by leaving the Roam asleep…BUT…
I have seen some in the community here say they want a simple ‘power off’ option and not a standby option. I guess that’s a fair ‘ask’ but personally speaking, I would find that rather inconvenient for my own use-case.
One area I think could save a little more power however, is after the Roam has been ‘active’ - When the Roam has been used/played-to by a user, it does not fall-back to sleep-mode for 30 minutes (same as the Sonos Move) I think that time period could perhaps be reduced.
I fully understand why 30mins was chosen, as that caters for the case where a user pauses their speaker to answer the front door or take a phone call etc; but I think most such interruptions would be catered for by using a 15 minute period instead. Either way though, I can only see powering off the device completely as providing any real significant battery saving. The fact it’s a lithium-ion battery means it will lose some charge anyway - I would expect to lose between 2%-7% charge per day when powered off completely, depending on room temperature.