[Just thinking aloud here, that's what a blog is for, isn't it?]
I like use cases because they are great for modeling scope.
Once you have a use case, with its basic flow and alternative flows, then you can discover the most important paths through the basic and alternative flows, and clearly mark out the key scenarios.
Each one of these key scenarios corresponds to a single test case. So each use case has one or more key scenarios and the same number of test cases.
These test cases are what the customer and the developers must write together based on a user story which is going to be implemented.