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If the diligent ones don't outperform the 'chancers' then they don't get as good a grade yes, but if they can't manage that then they aren't brighter than the chancers are they? That's the point of the exam to test performance not effort.
If the exam is challenging enough to require more work, then longer term effort will be rewarded over cramming.
Similarly, if the exam is so easy that you can get a good mark with a few hours revision then you'd be pretty dumb to spend half the term preparing for it.
Is that the choice?
Kids who work diligently on coursework year round are rewarded. Not penalising children who achieve most of their performance in the exams doesn't disadvantage the 'diligent' ones does it?