The boaters hand book is not the byelaws, one was written by serious commercial canal operators, ( with very big top hats ) the other by CRT who know even less than BW, and they knew nothing.
closing gates was only invented by BW in the post war era when the system was going into deriliction, as an emergency measure to save water, CRT tell us now everything is all shiny and in perfect condition, so this measure does not apply anymore. They only keep repeating it because thats all they know, they can't tell you why.
I will admit some pounds have to have the gates closed, but only a few short ones, and i don't have a crystall ball so i leave everything open. If CRT sign posted the leaky ones i would happily close.