Assuming that the 757 Flight Management Computer functions similarly to the 737, this may be of help. For a VNAV descent, the FMC assumes that at it's calculated TOD, the throttles, sorry, Thrust Levers, will be closed and the aircraft will descend continuously at the specified airspeed until reaching any preset speed/altitude constraints. If the W/V isn't as predicted, this can upset profile maintenance. Within certain limits, the FMC will command the AP to vary airspeed to maintain the required profile, but if, for example, it is unable to re-acquire when too high even with increased airspeed, then that blessed DRAG warning appears. HOWEVER, far more importantly, if ATC don't give you continuous descent, then that carefully calculated profile is buggered, big time! How often do you level off whilst frequency is changed and further descent sought?
Seems to me that whilst the VNAV profile is good for economical descent guidance, it's far better to control it with V/S and LVL/CHG and keep just a little bit in hand for the inevitable ATC interruption to descent. Happiness is found beneath the glideslope.
