In the case of bike locks, it seems to me that the question is not so much whether the system *can* be defeated, but whether a bike thief will bother to defeat it. If there were tons of bluetooth locks all over the place, bike thieves would probably find a way to defeat them. If it's rare enough that most of the time when you use it you're the only one, it's probably a reasonable system provided that the actual shackle is also harder to break than average. But at that point, it probably isn't better than any other equally strong lock that has a key. Aside from those barrel-shaped keys (that Kryptonite doesn't use anymore) where you could open the lock with a Bic pen and no knowledge of lock picking whatsoever, most of the time when someone actually breaks through a bike lock they actually cut or pry it open. If the shackle can be quickly defeated with a hacksaw, angle grinder, crowbar, etc, it doesn't matter in the slightest how ironclad the encryption is.

Personally, I always have my keyring in my pocket and I haven't forgotten my keys in years. If I lose my bike lock key, I have also lost the key to my house and am therefore fairly motivated to find it (or not lose it in the first place). I'd be more inclined to worry about not having my phone, because smartphones are themselves often a theft target.