Carol well Cambridge have put the arguments from their point of view.
From a pupils point of view:
Having an opportunity to sample 4 subjects at AS gives you a chance to make a more informed decision on what to drop or keep on, in one DD's case, all four, in the other DDs case the subject she dropped was the one she originally most wanted to study. The decision at the end of Year 12 was different in both cases to what it would have been in Year 11.
In deciding between the A level and IB route it also means that a fourth subject in your bag as it were together with extra curricular helps match the breadth of the IB where some of the subjects are done at a similar subsidiary level. I agree having half a subject that takes you nowhere, leaving no potential to take it further, as seems to be the new proposal for the fourth AS ( but no one really knows) seems pointless, and it will no doubt wither on the vine. Internationally A levels are viewed as narrow and prematurely specialist, this is a step in the wrong direction.
In one DDs case her AS results completely changed her aspirations for subject and uni, since as it turned out she came into her own at the higher level. This I think is the unis argument, that as well as giving them a more robust indication of ability it encourages able pupils who may not up to now have realised how able they are, likely to be disproportionately those who have under priviledged backgrounds / poor schooling.
Since she has a learning difficulty (but is very bright) having all her exams at the end of two years would have been beyond stressful, and it is doubtful she would have shown her true ability. She is now doing very well at a top uni, where exams are at the end of one year, along with an element of constant assessment, so what would have been demonstrated by extensive examinations at the end of two years work?
The only benefit as far as I can see will be for some boys who tend to be less mature and over represented amongst those who underperform at AS and those with prodigious memories who thrive on examinations (which unis make clear are not what they want).
I am sure IB courses will now have students queuing to sign up. I am just glad my DDs are now safe from all the change and twisted thinking.....