Scott,
Largely because Right way up is right!
From the fuel perspective, there is a world of difference in the descent between having the thrust levers closed and one in which V/S mode is used. Thrust levers closed gives about 200kg/hour per engine. Give it V/S and you see at least 700kg/hour per engine. 20 mins in the descent gives a difference of 300kg. Multiply that up by a large airline number of sectors and you can see why they prefer us to use all the facilities available to us to reduce this. If I fly 4 sectors a day then that is a tonne a day saving from me alone.
Why did I write the last post? To try and get the area guys to give us a hint as to what we can expect... We can then do what you want IF we know. Fair? If I want to get the best from my crew then effective briefing is the key. I don't see any difference from the flight crew/ATC perspective here, the only constraint being frequency congestion.... And that is another HUGE issue!