If that's the case and Danielle can be ruled out as having made it, I would say the likelihood of a print lasting very long on a handrail (in a home with three children) is on the remote side, and that an unID'd print like that would, IMO, be very significant.
Personal observation: We have 3 boys under 10, and they and their friends rarely go up or down the stairs without sliding their hands along the handrail the entire way. Not just touch it for balance, actually slide. Since this would "polish" the rail on a daily basis and thus wipe clean any prints along the top of the entire railing, I would think that the unID'd print on the VD staircase would be very fresh, and like you, very significant. This print would indicate that someone other than the family and the pizza partiers was in that house very, very recently to the forensic team's work, and evidence of a stranger in the house (or at least unidentified person) around the time of the abduction ought to be of the utmost importance (IMHO). Why the LE are passing it (and the unID on the glass sliding door) off as irrelevant is beyond me.