At a CIA Forward Observation Post, who would cover the entrances? Certainly not the CIA Chief of Station or Agency analysts. That sort of guard duty is now performed by private security contractors, which in this case is Blackwater/Xe.
Perhaps, some of the Xe people in Khost did assume additional duties. But, there can be little doubt, failure to search the bomber led to the loss of US life. That unenviable role belonged to the hired security guards.
Don't try to deflect responsibility.
Here's a good explanation in the NYT of the role that Xe guards have in perimeter security, and how that role has expanded to CIA personal guard, and even some operational combat in the field. But, at a base, these guys primarily still do perimeter guard duty, which is the most exposed and potentially deadly job on base, and also one of the most important:
http://www.commondreams.org/headline/2009/12/11-0 In the spring of 2002, Erik Prince, the founder of Blackwater, offered to help the spy agency guard its makeshift Afghan station in the Ariana Hotel in Kabul. Not long after Mr. Prince signed the security contract with Alvin B. Krongard, then the C.I.A.'s third-ranking official, dozens of Blackwater personnel - many of them former members of units of the Navy Seals or Army Delta Force - were sent to provide perimeter security for the C.I.A. station.
But the company's role soon changed as Blackwater operatives began accompanying C.I.A. case officers on missions, according to former employees and intelligence officials.
A similar progression happened in Iraq, where Blackwater was first hired for "static security" of the Baghdad station. In addition, Blackwater was charged with providing personal security for C.I.A. officers wherever they traveled in the two countries. That meant that Blackwater personnel accompanied the officers even on offensive operations sometimes begun in conjunction with Delta Force or Navy Seals teams.
This raises the issue of whether Blackwater mercenaries should be doing any of this, if they are apparently unable to do basic security effectively.