I thought competitions sounded kinda dumb, too, but then I entered one, didn't agree with one judge's assessment and decided to be a judge. Now I see that the style guidelines are necessary to attempt to set an objectively determined criteria to what is inherently subjective. I have given beers a score that indicates that the beer is not adhering to technical style requirements, but then let the brewer know that it may be a perfectly fine beer, otherwise. I even go out on a limb to compliment the brewer, if it is particularly good, despite style issues. This goes back to the commercial brewers, who are freer in their categorization of beers and allow substantial stylistic drift.
Now when I enter a competition, I know that order of the flight, handling of the beer, pouring of the beer and even lighting conditions of the competition may have impacted any one score and I am convinced more than ever that packaging the bottle for a competition may be a huge difference between a beer that comes out of the gate well and one that appears lacking at first glance. For this reason, I try to give a beer a healthy chance and always go back to it before finalizing my bottom line number.
In the end, the competition should be fun for the entrant and a worthwhile experience for the judge. Be civil and honest and find something to encourage the brewer, if the beer falls short. New brewers appreciate the feedback and may improve their brewing more quickly as a result of good feedback; old pros know not to enter a beer unless it is pretty darn good and they do everything they can to make sure it will hold up through the process to get to the judge's glass in the best shape it can be.