The answers lie in durability, strength and contamination. Incipient (early) decay reduces wood strength and is easily able to reactivate to full blown decay; refer to photo 5. On site surface treatment only affects the very outer areas of a piece of timber. Incipient decay will remain alive but dormant in the inner areas until water vapour travels pass the treatment and promotes a 22 to 27 % timber moisture level; refer to Fig.1. Also moulds like Stachybotrys atra cannot be removed completely and will reactivate at similar moderate ranges of moisture presence. Further not all fungal strands will be removed even by extremely careful repair. These microscopic fungal strands of decay and mould can continue their cycle promoted by this moisture presence well below that required activating them originally. Therefore repair should aim for the best possible drainage and drying environment.
Problems with decay, after leaking, often start at the interface of the outer wood surface and the cladding. Moisture that gains entry tends to sit or is entrapped at this point. Drying is by slow diffusion and drainage which is often restricted; refer to Fig.2. By comparison walls with drained ventilated cavities allow freer drainage and building wrap drying 2.2 to 3.6 better than direct fixed claddings. Most importantly they separate the wood frame from the cladding; refer to Fig.3. Everyone knows that if a pair of socks when washed and folded and put away in a drawer instead hung out to dry before being put away would dry slowly and may end up musty. A similar comparison can be made between direct fixed cladding and well designed and built cavity claddings.
Target repairs have numerous localised cutouts in the areas repaired. This creates higher risk of future failure. These types of junctions are difficult to make weathertight; refer to Fig.4. This type of repair also leaves the cladding fixed directly to the wood frame. The inherent risk of moisture being entrapped at this critical point remains. The risk of reactivation of toxic mould and fungal decay is high. Drainage remains inadequate and drying is poor. Extensive testing has shown flashings are more effective with cavities than with direct fix cladding.
Also hidden defects are likely not to be detected. Decayed and weakened wood framing will be left and extensive bio-contamination can exist. Detection of decay requires years of experience in visual surveying backed up by microscopic examination by qualified persons. Even severe decay can avoid detection by visual examination; refer to the good wood hiding the decay from visual inspection in photo 5.