Answer: "It is not required but it helps..." so why not?
As someone said earlier this would depend on where it is being done. Every five minutes over Europe with Maastricht handovers is probably not the place. On the other hand what might help the next controller is to know what short-cut/direct routing a previous controller has put us on. This is hardly ever mentioned on the handover but amounts to a reclearance from the original flight plan. Aircraft are often heard over Europe being asked where they are tracking to but I don't ever recollect being asked for my squawk.
Why do people insist on requesting the Flight Level when asking for ATC clearance away from homebase? Very few Ground Delivery units around the network want to know what it might be - although as we know Dubai does. Why? It's just luck if we get the level we want and Bahrain etc don't seem to know what level we might have requested anyway, so apparently the message is not being passed along either. It just blocks the Delivery frequency with unnecessary verbage. If other ATC units don't want it why does Dubai? And when will Dubai get PDC/DCL?
And then there's Dubai Approach inbound...