Many confusing cross-threads running here, but a couple of points to make:
First, since the freeing up of the curriculum, there are quite a lot of honcho points to be had from children taking exams early. Not only does it look good in the prospectus, it also gives an easy "not quite ready" excuse for disappointing results and an equivalent "didn't we do well!" claim for average ones.
Second, very few teenagers realise that the law requires them to stay in school until 31 Aug of the academic year in which they turn 16. Many of those who are not deeply committed to an academic future begin to get itchy feet during Yr 11, esp if their 16th birthday is anytime before about Easter. I teach SEN and our children seem to do a lot better in their Yr 10 exams than in their Yr 11 ones.
Thirdly, the real challenge of GCSE used to be the ability to juggle an amazing number of very different balls simultaneously: actually, a really good "life skills" predictor: 10 exams in Physics, French, Marths & Music etc is a useful comparator if all taken at the same time, preferrably at the same age. But if they are taken at different ages, or over an extended period of time, then the objecive value of this is lost.
Fourth point, very important, will have to wait as DH is making eyes from bottom of stares.... (Freudian, sorry)