I understand it is a particularly special SSSI as its the only one in the country with a uniform slope extending for 360 degrees so it's particularly interesting for research. I can well understand people climbing up using lots of different routes is damaging.
I agree. I'm not of the opinion that people should be 'officially' allowed onto Silbury, or that people should climb it at all.
There is too much damage to be done simply by planting your feet on it imo.