I find them helpful for what is allowed in the style, but not necessarily considered to be "required", as well as those aspects that are absent, because they are supposed to be absent. Keeping all of that in my head is the problem. You must be pretty good to work without the style guidelines. I suppose iit comes with time, but when entering a beer and getting dinged for a characteristic that is perfectly permissible, one wishes that the judge read the style guidelines more carefully....just sayin. Again, I am sitting for the taste exam in June, so maybe I will come away with a different perspective, but I doubt it. I felt the same way about the bar exam when I took it and passed. And still feel the same way about it.
They certainly are useful, but knowing the guidelines is a fairly small part of passing the new tasting exam. A passing score requires mostly accurate tasting skills, ability to describe what you perceive, and high quality brewing feedback.
Consider that the score is based on five areas, each worth 20% -
Perceptive Accuracy: How much does what you perceive match up to what the proctors and other examinees detected.
Descriptive Ability: How well do you describe your perceptions? Lots of adjectives, level descriptors, etc?
Feedback: Do you offer high quality brewing feedback that matches the errors you've noted.
Completeness: How complete is your scoresheet - all attributes addressed, checkboxes marked, etc.
Scoring: How does your overall score match the proctors?
Knowing the guidelines counts mostly with score and to an extent for feedback, but it has nothing to do with the other areas. The minimum Scoring score is 9 of 20 points, so guidelines affect 30% of your score at most. And even for feedback, if you detect diacetyl and give good recommendations to remove diacetyl. You'll probably get partial credit even if diacetyl is acceptable per style - because they want you to show that you know your stuff.
I'd also add that I got a 77 with a general understanding of beer styles. ie - I know what they should taste like but could not quote specific attributes. That memorization is much more important to get National scores - especially for the written exam.
There is also language in the draft 2014 (now 2015) guidelines that warns against getting too picky about specific style points. As Gordon Strong put it "You should be able to reward a good beer".
The other reality is that although many judges refer to guidelines during competition, there is no time to read them thoroughly. You must have a baseline understanding of the style going in. Even with time to read a full guideline - a judgement based solely on comparison to a written description is likely to be poor.