Great thread!
You ATC'ers seem to be saying that after you give an instruction like:
"ABC, when ready, descend to 7,000"
pilots should not read back the "when ready" words; i.e, we should just respond:
"ABC, 7,000".
But, if there is no read back of the "when ready" part, doesn't that open the door to the kind of uncertainty that the "left XXXX" is meant to resolve?
For example, consider this scenario where VH-ABC is given descent clearance "when ready" but doesn't hear the "when ready" words:
ATC: "ABC, when ready, descend to 7,000"
ABC: "ABC, 7,000"
But VH-ABC, having not heard the "when ready" words, commences descent immediately and as seems to be customary when given immediate climbs/descents, does not add the "left 7,000".
So, in this scenario, ABC is now on descent but ATC thinks he is still maintaining his original level. What if the ATC radar was to now fail?
In fact, isn't the whole reason for reporting having left a level so that ATC can update the manual record (formerly the "flight strip") for just in case the radar fails?