One buddy system rule does not fit all circumstances
I would disagree. The rule "do not go anywhere without a buddy" seems to speak to every circumstance.
To think a scout should have less restrictions during the day then at night, plays to the misconception that light is good and dark is bad. More car accidents take place during the day, most kidnappings take place during the day, more home accidents take place during the day, more drownings take place during the day, and child molestation can happen anytime and anywhere.
This fact was driven home a few years ago when a local scout troop was having swim nights once a month at the local YMCA. Two scouts were being molested each time in the locker room just a few feet away from the troop's adult leaders by an assistant scoutmaster. The boys were not required to take a buddy to the bathroom because the SM was right outside the door.
Do not let numbers, or proximity, or a sense of security fool you. What decisions you make to protect your own child are up to you. But as Scout leaders we have a legal and moral responsibility for other peoples children. Always using the buddy system is the easiest to do, easiest to teach, and easiest to learn method to help us do that.
Having a really good excuse not to do it will not protect the scout, nor does it release us from or duty as leaders, and it certainly will not appease the parents of an injured or missing boy.