I agree on the subject of bad exam questions. For example, in philosophy exam, one of the 15 markers asked to “Outline and illustrate the view that certainty is confined to introspection and the tautological”. We were never taught these terms in class (hell they weren't even on the revision guides I read), but I was lucky enough to know them both and deduce they were simply asking for you to define analytic and synthetic truths. Most people were clueless, which is to no fault of their own. After I told them, they all were filled with frustration as they knew they could of answered the question and gotten the 15 marks. AQA Philosophy is an example of a subject riddled with piss-poor mark schemes and structure, to the point where our teacher (who is an examiner herself) basically told us that it's an unpredictable, wildcard subject. Alot of the people who didn't get that question were A grade students, and the ambiguity of that question may be the decider between an A and a C for them.
For subjects that require memorization of facts as opposed to creative writing, History and Psychology are examples of subjects with such big syllabuses that it's near impossible to memorize everything. You can always get one exam that emphasizes all your weak points and emphasizes the strong points for the guy that didn't revise at all and he gets the A and you the D. Similarly, you can get an exam that asks for all the points you revised hard on, and you come away with a 97% overall. When you get the former, having the option to retake gives you an opportunity to get a grade that truly reflects your ability.
This new system means that the ingenious chemist is denied the chance to go to Cambridge as a result of bad luck, but the mediocre one does. Maybe the example is exaggerated, but the point embedded in it remains.