The fuel nazis have had a recent purge on using this function in our mob.Smoother transition/engine cooling etc are based on pure urban legend and "Heard it from a friend" sources so they've got a small point I suppose (although the boeing can be a bit "pitchy" in vnav .)
The best reason I often find for using it is "watched kettle" syndrome. The "reset mcp alt" prompt appears at tod/-15 and in a busy atc environment the request for descent is usually made upon this prompt. Assuming you managed to get the call in quickly, the aircraft will then chunter along for about 2 minutes( ..one mississippi, , two mississippi,,count to 120..)) before it commences its descent on the vnav path. I often imagine the controler in a busy sector looking at the block and wondering why the hell we are still sitting at cruise level when we just barged in on a busy rt stream with a request for descent.
When does "at pilot's discretion" just become "when the box decides to get on with it"?