Its not true that this rule is only for long haul pilots.
It is also used for preventing fatigue in short haul pilots.
It prevents those lovely rosters you get whereby you work:
Day 1. 0600 - 1900
Day 2. 1700 - 0500
Because the time between finishing first day's duty (1900) and starting second days duty (1700) is 22 hours. So, you could start work before 1300 or after 0300.
Thus preventing the loss of a night's sleep.
Its an excellent rule.