A video was a convenient excuse. Since riots over "flushing the koran" report and protests over cartoons had occurred just prior, rationalizing a terrorist attack on a U. S. consulate in a dangerous part of Libya as a protest gotten out of hand because of a video nobody had seen was a convenient excuse, easily believed by the low information crowd.
@bn13814 :
The video was convenient for the US to use, and it was convenient for the attackers to use as well. One of the attackers admitted that the video was used as a catalyst to help them implement their plan. IMO, the video story is an over-simplification of one component of a much more complex situation.