Just got bit by this one.
As previously mentioned (Feature Request - Purchase Order Number entry for any payment type
), but then put on indefinite hold. PO Numbers are a requirement for most orders in business environments regardless of the choice of payment method. They're not a payment method (the payment method would be "Net Terms" or "Account Credit").
It might be confusing for consumers to see this field, but it is a must for businesses. Not only to capture it, but to display it in the order list, and to be able to search orders by it (so burying it in an XML field is probably a bad idea).
IMO, It is shortsighted for nopC to leave this to customization, or to consider it a low priority/impact feature (when it is a bare minimum requirement for a platform that could be used in a business environment - and having a well defined convention is important).
If we want to hide it for consumer sites, it could be store or role specific (ie, turn it on/off either for the whole store or only for a given role).