We are a forces family. My dh boarded from 9, my db boarded from 11 and I boarded at 6th form. My 13 year old nephew boards (db also forces) as will the 10 year old when he gets to senior school age. DS doesn't board as the MOD picks up the day fees in Brussels, otherwise I couldn't have moved here to be with dh.
If the concession on day fees here were to be withdrawn, then ds would have to board either at a British Forces school in Germany, or back in the UK.
My dh changes appointments roughly every 18 months to 2 years, so it doesn't take a rocket scientist to see what that does to continuity of education, especially as you sometimes don't know where you are going to be sent until approx 6 months before you move, and sometimes you are given less notice than that. You may only know the area you are going to and have no idea about where you are going to live, or the schools available until 2-3 months before the move. I know one Army family who are due to go back to UK in August, having been posted out of UK since 1997, and they still haven't been allocated a married quarter. This will be their 14th move in 17 years...not good for options and GCSEs really!
The alternative is to weekend, and after 4 years on the trot it can get wearing. Sometimes it's a choice between your marriage (being able to move and be with your dh), or your children going to boarding school.
Having said that, if we were still in the UK, and being a Navy family, we have our own house, and I have til now not moved; I would have considered boarding school v seriously for ds, as he is an only, and being with the other kids would be good for him.
Like other people I know of families where the mum has died young, dad is in the RN, and the kids have had to board as the dad has had to go back to sea. My godson went to board as there was good provision for his particular SEN, and my god daughter went to the same school.
It depends on the child, mine doesn't want to board, and I'm not going to force the issue, but there may come a time when I consider it to be the best thing for him, and he will go.
I'd be interested from an earlier post whether forces families are counted as serial expats or dysfunctional - both labels which I find offensive, as we are families posted around the UK and abroad who are trying to do the best for our kids.