I also don't understand the weird rules key fobs and locks have, the states seem totally divorced from the real world.
But part of the nice thing about the app is that there's no cost to having extra "keys," so there's no reason to not have the app for your wife's car on your phone.
I would have to have Mercedes.Me service (which we do not) and be willing to let them spy on everything we do. No thanks.
When I press unlock on the fob for my 2001 car, it unlocks unless the battery is dead. I can even reprogram it for two brand-new fobs without going to a dealer.
But part of the nice thing about the app is that there's no cost to having extra "keys," so there's no reason to not have the app for your wife's car on your phone.