Level 4 new-ish player runs into a water hazard for the first time. "Hey cool, I got this neat ring of waterbreathing in Korthos. I'll just use that!"
Clicks ring on the hotbar to equip
Clicks ring again to use
Clicks normal ring to not be fighting with a ring of waterbreathing on instead of the awesome Acrobat's Ring
"Hey, where'd my waterbreathing go?!? This game sucks!"
Uninstall.
EDIT:
Look, I'm not going to claim that this scenario is likely, but the fact remains that triggering a spell via a device should not require that the device remain equipped. Spell durations have a meaning in the game, and applying waterbreathing via a device is no different than applying waterbreathing via a spell. The spell duration is the key factor, not the device. The spell should never just end because the device that cast is is now in the backpack of the user.