Well I don't have the expensive belts most of you have... I did check with a friend and a mysteel belt is twice as thick as mine. Mine is twice as thick as the very cheap ones...
I have had three times where the metal broke (metal fatigue) in some places that poked me yet I could still not get out. I needed a key. (Not to mention a few loose screws....) So yes, I have had several belt issue emergencies.
Not to mention the one time my back went OUT. Swelling and pain required immediate release. I was downstairs and the only key was on the 2nd floor, and I was alone. Let's just say, I did not plan for that.
So as for me Emergency keys are required. How that works for others....