This is a practice that dates back to at least the Second World War. It's not anything new. It's something that we've done, and, honestly, the philosophy hasn't changed much. But it emanated from the experience in the First World War, when they really asked much more of human beings than they ever could do, until they finally couldn't take it any more. Then they took them out of this intolerable horror, and for some reason they didn't want to go back. In response to that, what was driving this at the time was not a concern about their long-term mental health, in part because we really didn't think in those terms; what drove it was an operational necessity, which was an unsustainable practice. In World War II, first of all, they started rotating people out of the front so that they didn't have to go crazy first, before they could be excused, if you will.
That's what has been going on, and I don't know if there's any research on it, but it's generally accepted in military organizations that this is the right way to do it when the goal is operational effectiveness.
There is very little research on what the mental health effects are of that practice. There's one little study in Israel that seemed to suggest that, if anything, it was more good than bad. But that's about it. It would be a very difficult study to do.