They wouldn't, but they should have. Usually, because witness identifiactions are not wholly reliable, especially in a case where the alleged victim was under the influence and/or has at least twice failed to pick the person out of previous line-ups (be they standing or photo), a good investigator would confront the suspect with the identiofication and ask them if there's anything he (the suspect) wants to tell him, and then escalate the interview from there to an interrogation. A good investigator would seek any alibi the suspect might have, and then work on verifying or eliminating that alibi in order to determine if this was a viable suspect or not.
You see, the fact that none of that was done clearly demonstrates that all they were looking for was a prima facie case so they could indict anybody who was warm and breathing instead of the right person (assuming that a crime had actually occurred, of course). By not interviewing him, they relieve themselves of the necessary follow-up so they can just go with the identification to present to the Durham Grand Jury. It's then their burden to decide if there's enough cause to believe there was a crime and that the persons named in the indictment are more likely than not responsible for it. That way, Liefong and his cohorts have a panoply of other people to blame, and we already see that coming out in Baker's recent statements that blame Mangum and others for lying to the cops. She's one (deserving) scapegoat, and the grand jury will soon become another.
All Lifong had to do was ensure that professional investigative steps were ignored in order to get the indictments with a straight face. Grand jurors are notoriously dumb and clueless about the original purpose of grand juries and what their role, responsibilities and powers are. They're usually a bunch of dumbass bumpkins.
One thing I hope comes of this case is a media investigation and public airing of the (indicting) grand jury process so that people on future (indicting) grand juries will come to it with an understanding of what they are really there for, and it certainly is NOT to serve as a rubber stamp to the Mike Nifongs of this world.
Otherwise known as "plausible deniability".