I mean, I get a full week from my coros pace 2, with 5-6h of GPS cardio tracking (running) and 24h metrics (steps, stress, sleep, etc.) on a 310mAh battery. It takes a whopping 2h to recharge back to full, I would hate having to manage a tiny extra battery to save those 2h of not wearing my watch.
These watches typically come with charging cables, not a docking style station that you put them in. And keeping devices at a perpetual full charge for expended periods of time is a surefire way to kill the capacity quickly.