The difference is that there are more lets on serve's points than during an exchange because the players have all the time they want to play with precision and with little margin between the ball and the net. So the proportion of net is higher. Moreover, a net on serve would be always a point for the server (otherwise the ball is out, and it's a second serve) whereas a net in an exchange can go to both direction (the hitter or the other player). So it's more unfair. (not to mention that this new "rule" is totally USELESS)
Always point for server? Hardly. Most of the lets I see are of the kind where the ball trajectory remains relatively unchanged. The amount of points where the serve would turn into a perfect drop shot is low indeed. With a high bounce on the net it might even set up the returner for a winner, provided he's quick on his feet.