Garrett Epps, writing for the Atlantic:
Fingerprints and photos are taken to make sure that an arrested person really is who he or she claims to be, and to make a quick check of the records to see whether there’s a criminal history there. Is a DNA sample really a part of the identification procedure? Or is it a free-form investigation of whether the person may have committed some other crime?
Fingerprints are questioned less, in part, because they have been used in criminal investigations for ages. DNA hasn’t. Plus, there’s the issue of DNA’s perceived infallibility. Yes, it’s better at identification that fingerprints, but it’s not as foolproof as some people assume.