In addition to Rule 9.17(c), which you've quoted, there is also this comment from the rule book:
Rule 9.17(c) Comment: The Official Scorer generally should, but is not
required to, consider the appearance of a relief pitcher to be
ineffective and brief if such relief pitcher pitches less than one
inning and allows two or more earned runs to score (even if such runs
are charged to a previous pitcher).
My understanding is that the "should" generally outweighs the "is not required to." In addition, I think that MLB scorers have additional guidance on when to use "brief and ineffective," such as if a pitcher pitches a full inning and gives up at least four earned runs. MLB scorers gather regularly and have some pretty specific training so there is uniformity on how situations are scored across the league. As I understand it, they have generally been given firmer and more complete direction on when to use the "ineffective and brief" situation.
There are some wrinkles, however. When a reliever is ineffective and brief, the win has to go a subsequent pitcher (who has to have been more effective). Thus, if a guy is ineffective and brief in the top of the ninth and blows the lead, but the home team rallies and wins, that guy will still get the win, because there was no subsequent reliever.
Another wrinkle is that this rule can deprive a guy of a save. Last year in a Saints game, there was a situation where an opposing reliever was ineffective and brief in the bottom of the eighth and blew the lead. The opponent rallied in the top of the ninth and then their closer did his job in the bottom of the ninth.
The guy in the eighth SHOULD have gotten the win, EXCEPT he was ineffective and brief. The guy who followed him in the ninth therefore got the win, even if he may have preferred to get a save for his stat line. However, declaring the winning pitcher takes precedence over awarding a save.
Because the standard is so tight, "ineffective and brief" is rarely used. Scorers don't really have discretion on it.
There is a place where scorers do have discretion, and that's where the starter doesn't go five, but leaves with the lead that is never relinquished. In that case, the scorer does have the ability to pick which reliever is deemed "most effective" and award the win to that person. If there are two that are equal in their effectiveness, it should go to the one who pitched first.