Why would you want to select v/s in the cruise?
Normal FMA's for the cruise are SPD / LNAV / VNAV PATH, speed is controlled by the auto throttle.
If, as you say people select v/s in the cruise the FMA's will be SPD / LNAV / V/S,
speed is still on the auto throttle.
All you are going to do is encourage altitude excursions.
