I tend to agree that the actual code writing phase is easy if the spec is very thorough. But the spec can never specify everything and tons of little decisions

I tend to agree that the actual code writing phase is easy if the spec is very thorough. But the spec can never specify everything and tons of little decisions are left up to the developer. There are always people on the team who will create little messes. Those messes start to pile up unless there is a very strong leader who refuses to let any inconsistent code through (and is given the time to do so)