@Laura I think this discussion is getting off track, I suggest we move it to https://unwedchastity.org/d/717-emergency-key
That said, here is my breakdown of all the pros and cons I can think of.
Pro:
Reduces the chance of injury as a result of the belt (admittedly a low probability event but the consequences are potentially quite high)
It makes it less likely the belt will be damaged or destroyed if it needs to be removed in an emergency. (if emergency services need to cut it off, you will be out the money spent on it. Not as important as your safety but worth considering.)
If you need to persuade someone that you are wearing voluntarily, it will be a strong argument to say "I have a key for emergencies so I could get out if I wanted to".
In the worst-case scenario where you need to escape your keyholder, an emergency key makes that much easier. Also, a keyholder who knows about the emergency key might be less likely to overstep than they would otherwise be.
Con:
It will take willpower to not use the emergency key when you shouldn't.
The tamper-proof container is another potential point of failure of the system since security is never perfect and you might find a way to access its contents covertly.
If your motivation for wearing is about feeling secure, having an emergency key might interfere with that.
Overall, I think the balance strongly favors having it rather than not but I can see a scenario where the cons would outweigh the pros.
