It's easier to have strong rules and allow ROs some exceptions in particular weather conditions than have looser rules and expect ROs to augment them.
If you know for race X that full body cover is required but conditions on the day mean that the RO exempts overtrousers for instance then it's a lot simpler to turn up with all the kit required and leave the overtrousers in the car than have to scrimp or buy extra kit at the race.
