The phrase "best in slot" has taken on an almost untouchable mythos. It's not hard to see why. The experts spend a large amount of time crunching numbers and evaluating items for every spec of each class. They write software just for the purpose of telling you what items are better. When those people say that an item is the best in slot, that's the one. That is the item for the slot, and no better item exists. Anything else is just a placeholder until you get the best.
Of course, a lot depends on what other gear you are wearing. Even so, as raiders, those best-in-slot pieces are highly desirable. But what if our best in slot isn't our main armor type?
Recently my guild has gotten into what's turning out to be a hot debate.
It all started when a Paladin and Shaman both rolled on a mail belt; now this item was a best in slot piece for the Paladin but the officer automatically dismissed the Paladin's roll because "Paladin's normally wear plate and Shaman normally wear mail" and this was the only reason that the Shaman was awarded the piece over the Paladin. The pally did state before it was awarded that it was a BiS piece for him, but that fell on deaf ears.