The fallecy of concatenation

Whenever I discuss configuration management with anyone that is new to the concept, and even some people that have been doing it for a while.  There’s one concept that comes up that I have to argue with people about incessantly.  It’s this concept of concatenation.  Basically what people want to do is have this stub of a file be global, this other stub only effect this particular subset of machines, this other stub affect this other subset, then finally a stub that’s host specific.