The guys and girls at TC want us to set the next altitude constraint on the MCP and not the final altitude on a SID with a stepped climb.
On the TC radar display, the altitude you set on the MCP is displayed on the radar display in orange alongside the traditional radar label, thanks to Mode S. The controller doesn't know what is on the legs page of your FMC and they can only see what we set on the MCP. Therefore they can only assume that what is set on the MCP is the level we are climbing to without any recourse to level flight.
To give an example of why not to set the final altitude on a stepped SID. For arguments sake, we are on westerly ops and we have a departure from LHR on a Detling SID and a LGW departure on a Dover SID launching at similar times. The Dover SID from LGW is stepped initially to 4000 to keep us below the LHR departure that will climb to 6000. Should we elect to set 6000 in the MCP when leaving LGW, then the TC controller will see two aircraft turning towards each other and both climbing to 6000 feet.
I suspect the LGW departure will be challenged PDQ by the controller.
As an aside, I fly the 777 and at my outfit, we are expected to set the next constraint in the MCP until it is assured or we have a ATC clearances that supersedes it. My own view is that even if in level flight, the compliance of a constraint can only be assured as you approach the waypoint with the associated constraint. That is when I would be expected to set the next altitude.
The above may be overlooked if the waypoints are very closely spaced.