One reason that we sometimes give restrictions other than what is on the STAR is due to traffic. You come up with what you need to get aircraft safely by each other and not have to continue to monitor one aircraft. It allows you to go on and continue to watch everyone else and move them about also...
As to the aircraft rates <G>... Well if you are in an A340, then we shouldn't expect much out of you <G>..... As to the shorter Airbusses. They normally seem to climb rather well..
regards