I have been thinking about this, and have to agree that with todays crowded skies, descending without clearance could put you in "grave and imminent danger".
-You don't really have any way of assessing that danger in advance (you don't have the traffic picture below you).
-You can't respond to the TCAS god when he commands "climb, climb now". That will definately upset the other guy descending, descending now.
Maybe some companies need to update their SOPs?