Guess Himan could've been there for his comments in his notes that Nifong wanted the Cabbie arrested and wanted to know when he was in custody.
What excuse for Clayton ? others?
Did Clayton perhaps serve a warrant on Elmostafa?
IMO, they were there to let the cabbie know that they will be watching him no matter what.
Not much point in intimidating the cabbie - both sides have his sworn statement, which should be admissible in court without the cabbie's authentication. And they have the timeline of the cell calls / ATM withdrawal / fast food pickup / dorm entry...