As a mere PPL I tremble to post here, but aside from all the discussion about what is or is not legally required, I have to ask what is the actual purpose of the readback? It is to ensure that you have copied ATC's instructions correctly.
You read back the FL instruction to confirm that you are going to the level you have been assigned. When you are heading into some of the most congested airspace on the planet, and ATC are feeding you into the traffic flow, don't you think it's equally important to ensure that you have copied the speed correctly?