20 feet is about 1.75 seconds at 7 minute pace. Are you saying that you can absolutely be within 1.75 seconds either way of a specific pace every time?
Because GPS really is that accurate. A better argument against GPS is that they are not very instantaneously accurate. If you ran a perfect 7 minute pace, one moment it will say 7:15 pace, then 6:45 pace the next moment. This could throw you off quite a bit if you were relying on it for instantaneous speed and trying to run perfect splits.
But if you wanted to use your watch to help you find the right pace, it could help you find it by trying to hit that pace while ignoring your watch for a minute. Then look at your average pace in the pace minute and make your slight adjustment faster or slower as needed. For a road race, this could be helpful, since you likely don't have quarter mile splits to verify pace quickly.
At any rate, the OP used a false premise. Nobody "frowns upon" the use of a GPS watch, some people just don't want to use one themself but could care less if somebody else likes using one.