I haven't read any of thread, but these are reasons I prefer some 1-2 yrs "early" sittings for GCSEs:
The English system is so focused on GCSEs as all or nothing results, getting one/some over & done with proves to kids they can do it and it's not as overwhelming to tackle the others.
Multi-yr takes prevents certain time table conflicts that used to be very common and prevented study of certain combinations (e.g., so many people have told me they HAD to choose history or geography, never allowed both).
Vast majority of Unis or other destinations really don't mind & now expect staggered times.
It allowed the opportunity for retakes before the start of 6th form.
Some MNers have admitted they did bugger all work for 4 yrs & crammed at the end to get passes, so the all-at-end system did not teach them good study habits or give them enough feedback that really seemed to matter.
Completing all in a very narrow time frame means that if they have a bad spell in life for some other reason (hormones, love life, parents, whatever) that all their marks are screwed up, instead of just some. Spreading GCSEs hedges bets against the vagaries of teenage life roller coasters.
Plenty of kids won't get much better marks by waiting until last month of last yr, especially with so much pressure on at once.
The old system was too much pressure for many.
I could probably think of more if I allowed another 10 minutes of thinking. Want to read what others wrote, though.