I suppose the point I am making is that while you may cry 'But they should be spending that time doing handwriting!', for some children that handwriting is NEVER going to happen unless you get the engagement with the schooling process first.
Get a positive attitude to school, and the handwriting can follow.
If the 'engaged' children don't get anything directly out of Forest School, the indirect benefit of having engaged fellow-pupils is almost certainly worth the time investment.
That said, I do think it is mainly a KS1 / early KS2 thing, though a good FS facility can be used to teach Science, Art, setting work in English etc intermittently in higher years.
Yes, children CAN do the same at Cubs / Woodcraft Folk etc. However, the families for whom FS has the greatest benefit are almost certainly not those who get their children onto the waiting list fr Cubs at 4, who will buy the uniform and sew on the badges, who will get them to meetings every week...