Many times people don't spend the time or effort in acquiring information and rely on experts to give this information. And the argument from authority works when (from Wiki, my school text says the same):
The strength of this authoritative argument depends upon two factors:
The authority is a legitimate expert on the subject.
There exists consensus among legitimate experts in the subject matter under discussion.
What if the subject is under question. This is usually done from a "bottom up" approach about the evidence, but that is sometimes insufficient as not everyone has the resource to check for themselves. I think there is/should be a "top down" approach, where the subject is open to outside criticism from experts of closely related fields, the agreement to consensus is proportional to the amount known or time studying, and there would be no divergence in opinion (ex: chemists learning about "argument" between pharmacology and homeopathy would tend to disagree with homeopathy). Are there any other points or factors to prevent an unquestionable authority?