It was apparently an issue in Vietnam, where individuals were rotated in and out of existing units. Units that had bonded over training, death, and suffering -

It was apparently an issue in Vietnam, where individuals were rotated in and out of existing units. Units that had bonded over training, death, and suffering - and then a new guy (sometimes a new leader!) shows up. From what I read, people were not always welcoming and this was also a way to avoid becoming attached until after that first battle