The failure case is still terrible and consists of me with a hacksaw at 3am on the side of the road "Stealing" my own bike.
a final note, the battery innovation does not even have to be that much of a breakthrough. if your phone lives for a couple of days on a single charge, then the probability of running out of battery will decrease dramatically (since most people charge at least once every two days).
Feel free to use something like this. It's geeky cool. I've just been burned by things like not being able to contact my ride from the airport, or my phone randomly deciding that it's just too dead to turn on at 8%, that I'm not trusting it for keys to my transportation / apartment.
For those things, if they have battery, like say my apartment combo lock, I want things with a good battery life (in the years) for the whole thing, a good failure state that I will always have on me (a combo), and obvious notification EARLY when the battery is going low.
There will be improvements, but they won't be easy or particularly fast.
edit: Didn't notice you were OP. If I were to use this product and my phone were to die, what could I do?
Also you can apply the same argument to Lockitron. They can grant access to anyone to open your door lock and I believe you have more valuable stuff at your house.
More convenient, perhaps, but now in addition to jamming a pen into the lock or cutting the chain with a pair of cutters (or whatever myriad ways to physically assault the lock that there are) you have to worry about potential software exploits enabling remote entry from anywhere in the world. It seems to me that these things are, practically by design, exponentially less secure than their purely 'analog' counterparts.