Bonjoy:
Hmmm, it's a different force acting on it. Usually holds with lots of foot-traffic become polished (unless the outer layer is removed such as Hanging Arete at Higgar) because pressure is exerted downwards and the pressure is spread out over a wider area whereas brushing exerts downwards force and force backwards and forwards with brushing and is localised in a small area. Look at New Jerusalem for example, nobody stands on that hold and yet it's become massive through overbrushing.
Sorry Andi but you are failing to see the wood for the trees. Pressure on footholds is multi-directional for various obvious reasons (do I really need to go into those reasons). Rotational force is particularly common on footholds and especially damaging. The difference in area over which load is spread is irrelevant as it is hugely outweighted by the difference in in scale between the force. Let me illustrate this another way. Which would be more damaging to your teeth, me brushing them with a toothbrush, or me jumping up and down on them?
The bottom bit of the Higgar arete is soft damp rock and never had a resistant crust to speak of. As someone who does the odd new problem I can tell you that some rock (especially near ground level), is shit from day one. I've actually heard people blame the damaged footholds on this problem on brushing. Do these people never notice the sand on their boots every time they use such footholds?!*
Can't comment on NJ as I haven't seen it for years. Where a handhold is so fragile is crumbles under a toothbrush I tend to favour re-enforcement with superglue rather than a doomed to failure brushing veto.
* I sealed these footholds with rock sealant some years ago so they should be stable, but haven't been back to check in a long time.
