St martin: I would like to see how you manage to stay on the glideslope in an approach with fluctuating speed (say +/- 5 or 10kts). You will be all over the place if you chase the speed with pitch mate. The larger the aircraft the more obvious this becomes.
The simple fact is thrust overcomes drag which causes an acceleration and therefore a speed change. It is because of this speed change that the RoD eventually changes.