Peak oil and climate change aside...part of BMW's effort to make carbon fibre a mainline material for its mainline cars is to consider the cost of repairs.
Don't you think BMW has considered such an obvious point in their studies of using the material? They are not building cars in a vacuum and consider the marketability of the product as an essential characteristic that would make it a viable material for car construction.
With carbon fiber now being used (not without its challenges) on the Boeing 787 and major investment by several automotive companies, it should be obvious that with industrial production levels of carbon fiber will come substantial breakthroughs on cost.
While I'm not saying carbon fiber will be as cheap as steel (but, then again, who knows?), BMW obviously feels that by 2015 it can be competitive so that the advantages of the material outweigh the disadvantages. I'm sure they realize these cars will get damage, and few people would buy a 7 series if even a minor dent would cost the thousands of dollars it would cost today to replace a carbon fiber panel.