The explanation I heard is that the Boeing designed the V/S mode to be used during the old dive and drive type Non Precision approaches, and wanted the engines to be spooled up at low altitudes, even though it only comes up very little, barely a spool-up.
It is always a better technique to use VNAV or even the FLCH, because the V/S can fly you away from an Altitude thus get you in trouble!
Last edited by UAL320Capt; 19th Mar 2010 at 02:33.