One controller, only, who insists that "when ready leave control area on descent" is a mandatory readback. It's not a level assignment therefore it is my interpretation that it is not a readback requirement.
You are right, he is wrong, and it is a gaping cheese hole (Reason) in the way we assign levels. An aircraft at FL350 who is to be cleared to FL330 needs to have his intentions confirmed with a readback, yet the same aircraft cleared through every level below him in controlled airspace doesn't...
I have heard one (& one only) controller get around this (in the above example) by the following:
"Fuddpucker123, when ready descend to FL190."
"FL190, Fuddpucker123."
"Fudpucker123, left FL350."
"Fudpucker123, leave control area descending, Traffic blah blah blah..."
"Fudpucker123."