Colchester's "longest platform" is two platforms of roughly 12-coach length laid end-to-end (on completely separate lines - a little like platforms 2 and 3 at Woking, except they're linked via a crossover) so the sub-divisions issue doesn't really arise.
The longest physical platform is Gloucester, which splits into two sub-divisions - though its purpose was to handle two HSTs at the same time. Given the trains generally operate in shorter formations at Leeds, a split into more sub-divisions, though the physical length of the platform may be shorter, would probably make more sense than just splitting it into two or three.
All of Nottingham's platforms were/are split into sub-divisions (except for the bay platform 2) despite their relative lack of length (12-or-so coaches on most of the platforms, which were split into three sub-divisions, though the four-coach-length platform 4 was split into two sub-divisions to facilitate the simultaneous arrival of a train from Lincoln to Leicester and departure of a train to Leeds. Platform 5 (which had a Colchester-like arrangement with the "new" platform 4 so was short in comparison to platforms 1, 3, 6, and 7) may not have been split into sub-divisions (given it could only handle one train at a time, though it served both St Pancras and Cardiff services), but I seem to remember it was - though it is many years since I've lived in the Nottingham area and even more years since I've visited Nottingham station). Short the platforms may have been, though sub-divisions were necessary given most trains had either two or three coaches (and obviously the longer services would be announced as departing from platform 6 rather than 6b, etc.). Admittedly few trains used sub-division "c", possibly because the only permissive arrangements that usually occurred on full-length platforms involved trains arriving into the west "a" end, and possibly because the "c" ends were more exposed to the elements - though it was very useful to know precisely where on the platform the train would stop (something BHM could learn a bit from!) when I had heavy luggage!
Anyway, getting back on topic, Doncaster's quite difficult to change at for me - and platform 0 only makes things worse. And, twisting it slightly but staying with Nottingham, the interchange at Highbury Vale between the Hucknall and Phoenix Park branches is horrible - no wonder they advise changing at David Lane. To be honest, the Phoenix Park platform should probably be taken out of use - it's not as if having to change at David Lane will cause more inconvenience than working out how to find the branch platform at Highbury Vale!