I think the idea behind it is that everyone is in one way or another being affected by what is happening, there would be an overload of students insisting they were not "fit to sit" their exams (even if they were exaggerating). There was also a lot of pressure from students for universities to adopt a transparent policy that addressed their concerns especially after one university implemented the no detriment policy - it led to many universities to quickly follow suit.It is when you can no longer compare candidates academics. Trying to assess someone who has a 65 average across all modules sat (lets say six), versus someone with a 65 average based on a no detriment policy across 6 modules (when they actually have a 55 average), versus someone who has a 65 average based on 3 modules sat, versus someone who has a 65 average across 3 modules and based on a no detriment policy across those 3 modules (when they actually got a 55 average)...
To me, the fairest thing universities should have done was made everyone sit exams as normal, with additional reasons for "not fit to sit" policies. They could then apply the "no detriment policy" to any weighted modules towards final degree classification. For first and second years, this would have been pretty much what happens anyway, and the only people it would have impacted was finalists.
It just seems like an over complicated and lazy system, that encourages people to not sit exams, let alone take them seriously.
I do agree the policy in place encourages laziness and it means that those who have actually performed well could see their mark undermined. I think regardless of the policy it should have been made the same across universities as it seems those who have a less lenient policy will be disadvantaged.