In a race situation a beginner would have a hard time using the GPS in a meaningful way in my opinion. When surrounded by other boats they are the measurement on how your speed is. Having a GPS then is not really useful for a beginner in my opinion as he has more than enough to do with keeping track on what is happening around him and trying to get his boat going. A really experienced sailor would as you say not need the GPS to keep track of his speed, but for determining VMG and deciding on shifts, favoured sides etc. it could be useful. A beginner would not be in a position to really take advantage of this as he simply dont have the time to concentrate on this.

We have trained rather a lot with GPS units to try and find best trim. We have also raced with it in distance races but then mainly to find the fixed marks. Workes incredibly well in that configuration.

My personal opinion is that racing is a game and a large part of that game is determining shifts, favoured side, best trim, best VMG etc. etc. Sailing with a GPS removes some parts of the game which I wold rather keep. For training, I find a GPS useful, but a coach is infinitely better!