I use a wire brush sometimes to clean new problems - ie; rock that has not been climbed on before. Sometimes a nylon brush will not remove stubborn lichen and centuries of mud and crap from the surface of the holds. HOWEVER, I very rarely need to do this - most rock cleans up with a stiff nylon brush, and there is never any need to clean established problems with a wire brush. Whoever 'cleaned' the sloper on the Fin obviously had no idea of what they were doing, and now the hold is ruined.
When I do use my wire brush, I do so lightly, checking to make sure I'm not removing the patina from the rock - once this is gone you've ruined the rock for ever.
There is no need to have a wire brush in your bouldering kit unless you are establishing new problems, and seeing as only a few people seem to be doing this these days there should be no excuse for having one.
Daves point of some idiot having one in The Edge - if I catch them polishing my plastic, I will insert their brush where the sun don't shine!
If holds are caked in chalk, or have gone 'black' and greasy, wash them with a toothbrush and either water or isopropyl alcohol (degreases the rock but doesn't damage it in any way!). Never wire brush them - you'll only break the surface crust and then its goodbye to the hold for ever!
Isopropyl alcohol (or Rubbing Alcohol as the Yanks call it) is really good stuff (although very hard to find in the UK, you can but it in any Pharmacy in the US). Wash your hands in it before you climb and it degreases your skin. Clean your shoes with it and you will stick like shit to a blanket. Clean the rock with it and it removes grease and chalk, and also dissolves pof. Doesn't harm the rock, evaporates very quickly, and is a very good secret weapon in the boulderers armoury. :wink: