An example of when you don't need to call leaving but is consistent with the AIP is the frequency change that coincides with your ToD. When you make the intial call to the new frequency you just have to notify the assigned level that you are descending to. Some where in the mists of time there was probably no requirement to report leaving but an aircraft inadvertently left their altitude and ATC didn't realise. The solution was to require pilots to report leaving.