I'm not joking at all here. The rule you're talking about is a godsend, esp. for rubbish riders like me. The feeling of getting a 3 in a sections is so much better than a 5, and it makes all the sections attemptable in some trials, for people of any ability, for example, if i had to give up after 5 dabs (assuming you can still score no more than 5) then i would just not attempt some sections, because i would no theres no way i can get through in less than the required amount of dabs.
The rule encourages people like me to just have a go, rather than 5ing everything, or even asking for fives.
(I really have to learn when to end my sentences.)