A reader points me to this long tweet from Richard Betts, which I missed while I was away last week. It's certainly worth of reposting:
I see the '2 degree limit' as rather like a speed limit on a road - both are set by policymakers on the basis of a number of considerations.
On the roads, the main issues are safety, fuel economy and journey time. Regarding safety, driving at 5mph under the speed limit does not automatically make the journey 'safe', and exceeding the limit by 5mph does not automatically make it 'dangerous'. Clearly, all other rings being equal, the faster one travels the greater the danger from an accident - but you also want to go fast enough to get to your destination in a reasonable time. The level of danger at any particular speed depends on many factors, such as the nature of the particular road, the condition of the car and the skill of the driver. It would be too complicated and unworkable to set individual speed limits for individual circumstances taking into account all these factors, so clear and simple general speed limits are set using judgement and experience to try to get an overall balance between advantages and disadvantages of higher speeds for the community of road users as a whole. Basically, a simple limit is practical and workable.