I don't know much about how the intelligence community works, but I've got to think that they would place a lot of weight on statements from those people on the ground at the time of the attack since, after all, they were actually there. "Within 24 hours of the attack, both the embassy in Tripoli and the CIA station chief sent word to Washington that it was a planned militant attack."
http://bigstory.ap.org/article/libyan-witnesses-recount-organized-benghazi-attack
Yes they would put considerable weight on statements by people who were there, but that would still comprise only a fraction of the analysis. Eyewitness reports are frequently unreliable.
In my opinion, the WH and/or the intelligence community fucked up. If you think they were competent, that's your perogative. But I beg to differ...and to suggest that I have "pretty crazy standard for competence" for having this opinion is ludicrous.
I think the more you look into how intelligence works the more you will see the limitations present. There's a reason why the intelligence community never issues judgments that say "X happened". If it is certain enough you can say that, you don't need the intelligence community. When issuing judgments they use terms like 'high confidence' and 'low confidence'. (and as an example, the Iraq WMD NIE made several wrong judgments with 'high confidence')
I agree that they fucked up, but competent people fuck up sometimes, especially in a business that is as uncertain as intelligence.