Once cleared on the FMS Transition, there need be no further input from ATC in terms of vectoring, descent or speed. Although tactically some of these could of course be varied in respect of other traffic. Another possible angle is that they might help introduce a simplified Radio Failure procedure. If you know exactly what the aircraft's profile is then it would be possible to manage and position all the other traffic in front of and behind the aircraft which is incommunicado. Unlike today where you would clear everything out the way and try and guess what the pilot is going to do.
I have flown onboard a trial profile into Glasgow which with the co-operation of the Centre and Airfield ATC's meant that we had a continuous descent from Cruise Level all the way to 2100' on final. We entered the FMS Transition at the Terminal Holding Fix (LANAK) and were given no further ATC instruction till we called established on final.
Canada have also done a lot of work on these procedures and Air Canada now use them on their A320 and B767 fleets into most of the Canadian major airfields. There is a lot of interest in the UK and Europe now also and once we get the concepts proven then they will no doubt spread. In busy environments such as Heathrow though their benefits will be limited but I suspect will still provide some possibilities.