A good deal of ink seems to fly each year prior to Adepticon about one topic more than any other: the INAT FAQ. It is either viewed as the most comprehensive tournament FAQ ever produced, or reviled as a serious undermining of the rules system by which we play.
We've all come across that rule which is vague. It seems invariably that the rule that causes the most arguments is the one for which GW hasn't made an errata or clarification: Doom of Malanti's, Deff rollas (finally taken care of), the use of Descent of Angels for Storm Ravens with embarked squads, the Manticore's template scatter... Maybe those have come up in your games, maybe they haven't, but I guarantee you that in six months, these things will be gone over line by line by the FAQ council that Adepticon appoints.
But is that a good thing? Opponents have valid points. There are out and out rules changes in the INAT, no if, ands, or buts. This is not all from GW's canon. A group of people are putting thier own clarifications, changes and spins on the product that you've purchased. It's fan based, not offical. And there's the very valid sticking point. It's not official. Stelek, a well known opponent of the FAQ, articulated his views thusly: "You have no right to change the way the game is played."
Proponents are very quick to point out that even GW doesn't count its own FAQs as official, except for the errata. And GW's FAQs fall very short of answering questions that will come up in the higher levels of play, things that if answer one way or the other will determine who walks home with a pat on the back, and who walks home with a thousand dollars worth of prizes. Things like that can put real pressure on a judge to make the "right call", and no matter what call he chooses, the blood-crazed hounds of internet will savage him. The proponents value the transparency that the INAT brings; here's the call we made on this, it was put up months ago, please review it. At that point, there's no argument. There might be bad blood, or hurt feelings, but the matter is closed.
As for myself, I find the proponent's argument more measured. Having been to Adepticon but the once, I only found reason to call upon the FAQ one time. The judge came over and quickly applied the ruling needed with minimal fuss. I played seven tourney games under the FAQ, and really didn't have to consult it. Which made the sturm and drang over the FAQ seem wasted effort. Differences in play exist from person to person, region to region, and state to state. The INAT, or something like it, is needed at nationwide events to smooth out those differences. Is the INAT appropriate at every event, regional or otherwise? That depends on the payer base and the Tournament Organizer's call. The only reason I would be unlikely to use it at an event is player feed back.
As to the charge leveled that the organizers of Adepticon have no right to change or clarify the game to their liking, I would refer the gentleman to the gray box on top of page 2 in the 40k Rulebook, under "The Most Important Rule". The game designers want you to do with your game what you want, including making house rules and house clarifications if you see fit. The only difference with the INAT is its scope.