The biggest issue with cord blood banking is that the overwhelming majority of diseases that need to be treated with a cord blood transplant have a genetic component... thus, you don't want your own cord blood back that has the faulty gene that allowed the disease to occur in the first place, you need someone else's cord blood.
Second, most people are more familiar with bone marrow transplants/donation, where it can be quite difficult to find a match and your best chance for a match is a sibling. That is not true with cord blood transplants. The criteria for a cord blood match is not nearly as strict and it is much easier to find a match from a public cord blood bank.
Finally... much research is being done on harvesting and using stem cells from other parts one one's body. Cord blood is not the only place to get stem cells. There is one website that I know of where a guy had a stem cell transplant from Crohn's where stem cells were basically filtered out of his own bloodsteam and transplanted back into him- here is his website .
http://www.transplantforcrohns...plant_Procedure.html IMO, private cord blood banking is basically a scam that plays off parents' fears.