There were also protests in Tripoli that night. So it's not out of the realm of possibility to think that if the Embassy were left unguarded, that would have much worse consequences. Maybe that was the original plan - draw the Marine forces over to Benghazi so they could attack the Embassy in Tripoli.
Of course, we don't know. But your contention here is that the Embassy should have been left unguarded to go on a rescue mission, and hope to Hell nothing happened to the Embassy in the meantime?