The real reason as was told to me is the possibility of tail strike as the thrust increases inadvertently at low level if you drop below min spd.
This was disucssed at a Boeing conference our fleet team attended and the Boeing flight test guys were all convinced that "all on" or "all off" is the best way to operate the machine. The Boeing guys even demonstrated the issue in the simulator to the head sheds from the training department.
Not long after that our FCOM was changed to all on or all off.