There are many applications which are network transparent and would need extra code to know how to optimize when the other process happens to be on the same mac

There are many applications which are network transparent and would need extra code to know how to optimize when the other process happens to be on the same machine (e.g., using Sys V IPC or Unix domain sockets). There are good reasons to optimize the case of localhost. I am always bemused when people imagine that communication over localhost involves packets hitting an ethernet adapter.