Permission means that a nominal circumstantial clause may sometimes omit wāw. Preference means that an overt linker can be clearer because a nominal predication is capable of standing independently. Special cases include a bearer-pronoun as subject, a preposed adverbial, or a particle that already signals the required connection.
To compare the rulings correctly, analyze the surface syntax, the pronoun's referent, the type of predicate, the temporal relation to the main event, and the speaker's intended meaning together.