Moving one's hand from the thrust levers at V1 is symbolic in nature; it symbolizes that the point has been reached, previously decided before initiation of the takeoff run, at which the airplane is going flying should an emergency arise. Removing one's hand from the thrust levers is a form of commitment to the remaining takeoff; one is setting aside the decision to retard the thrust levers, and has committed to flying the airplane off the ground.
Keeping your hand on the thrust levers during an approach isn't merely for the sake of an autothrottle malfunction. In the event you need to continue the approach or make changes to the approach, you're the person manipulating those thrust levers. You're not so much guarding them in case the autothrottle suddenly retards them; you're there to take over, and at some point you will take over. In effect, you're doing the opposite of what occurs on the takeoff. On the takeoff, you have the thrust levers until the aircraft takes over (for those of you who use autothrottles for takeoff; I don't), or until the thrust is set for takeoff. On landing, if you are using autothrottles, the thrust is essentially set until you arrive at the runway (autoland excepted), at which point the airplane is given back to you.
On approach, the airplane has to do something; be it autothrottles of you, someone has to be manipulating that thrust lever all the time. On takeoff, conversely, the thrust is set at max or reduced, and you're not managing the approach angle at all, and your airspeed is manipulated by pitch. Set the thrust and go. Not until your initial thrust reduction during a standard noise abatement departure and then setting thrust later in the same procedure, do you again touch them. Not so for the approach and landing; the thrust levers are constantly being adjusted, or at a minimum gaurded to make an adjustment should it become necessary, either by you or the airplane.