Above should read: Sheriff's are NOT needed in court.
Deputy sheriffs serve as bailiffs in court.
Process servers cannot serve arrest warrants.
Sometimes the officer who serves a warrant is required to testify, usually at the behest of the prosecutor because the defendant said something incriminating when the warrant was served. In this case, I think Himan and Clayton were there because the defense called them in reference to Nifong trying to intimidate Elmo.
Any statement that a defendant makes against his own interests once he's in custody can be used as evidence. If he made an incriminating statement to the (non-sworn) jail officer who processed him (fingerprints, photo, etc.), you can bet that staff person will be called to testify.