The cord blood banking industry is right on the border between speculative medicine and outright scam. It's insanely profitable, which is why every doctor's office is littered with pamphlets for competing cord blood banks.
There's a vanishingly small likelihood that your child will have some otherwise untreatable disease that the cord blood will help with. Most of the things they say cord blood can help with (like genetic defects) actually wont help your child, since the cord blood has the same faulty genetics. The banks also tout the potential for cord blood use in future therapies. However, it's likely that any treatment that uses cord blood would be just as effective using stem cells.
So what are you banking, in this case? I have no idea. The cord blood might be helpful for your next child, I guess.
Another thing to keep in mind is in order to harvest the cord blood, you have to cut the cord before it stops pulsating (that is, before all the blood in the cord has reached the baby). There's a growing body of evidence that your baby benefits from this blood, and the cord should be left intact. So banking your baby's cord blood may actually harm your child. Of course, whatever the effect it's unlikely life threatening, but it does seem unnecessary.