To claim that Bong Sao should never be used is a complete heresy. It is a fundamental part of Wing Chun - why would it be stressed so heavily in Chum Kiu if it was not useful?
It all depends on the context in which it would be used. I can only assume the Opening Post is a kind of tongue-in-cheek joke.

Applications for Bon Sao are not allways widely known.

Here are some: