Salute!
I am not too sure about the Marines, but the Navy folks have scheduled cruises unless we go to war. So they know their schedule and they and families can love it or leave it.
The USAF fighter units are constantly going here and there without a long range schedule. Ditto for some of the tac airlift and maybe a small group of buffs ( the flying bomb trucks at the sandbox). The main airlifters are not going thru the same drill.
During 'nam we had a system of "overseas return date" plus "remote tour". So it was first in/out and then after all others did their turn, your number came up again. That was how I got my third tour in 1975. My second tour was only 170 days or so, meaning it didn't count for a "return date" or "remote tour". So early '75 I was sent back for the full "remote tour". The 170 day TDY option did not offer any financial help, whereas the PCS "remote tour" provided a few goodies for the family.
The situation the last 20 years has been about the same. Go, come back, go again, come back.......... I have heard there are some deals whereby you can make the jaunt count for a full-blown tour and then go to the back of the line after being transferred to a training outfit or non-flying assignment.
Oh well....
Gums sends...