Disagree on this. It has nothing to do with efficiency in context of unlikely events. As others have noted here, it is effectively an assertion of expected language/system/operating-environment properties. Think axioms.
No, I'm primarily thinking of "ain't got time for dat", as in "there's a very real deadline, I have a lot of other things to get done, and this case isn't ever going to happen and I don't have time to prove it to the compiler."
