Not a bad idea, but on its own, not worth the additional cost. It would only protect the battery in cases where someone steals it while off the bike—assuming the thief has a Bosch-equipped bike or intends to resell it, which seems unlikely. If the battery is on the bike, a thief would likely be targeting the entire bike rather than just the battery.
We've never had a battery theft among the many Terns we've sold. Whole bike theft? Yes. Bikes stolen without the battery? Also yes.
In a scenario where the bike is stolen but you still have the battery, does the old battery become useless if you replace the bike?