Muddy waters. What if (when) they fire a false positive?
At the moment police are relying on people saying "I saw some bloke that looked like that photo at this location", so there's not much difference.
You can show a person a photo, and then show them a "6 pack" and they can't find the person in the original photo. And many other examples. I'll try to find some cites to see if I've remembered correctly (and if the studies were any good).