First are they necessary? Very subjective. Removing the adventure of descending the Organ Pipes Gully is a negative in my book. I'm a great supporter of rap anchors for removing erosion pressure on gully descents but I think the Organ Pipes Gully isn't a great erosion problem.
Second : Increased congestion at the rap anchors below.
Third: Potential increased rockfall from people abseiling into the gully. This is the big one for me now that I see where they are. I'll try to observe what happens when parties use this descent. If, as I suspect, this causes rockfall down the gully then they probably need to go.