External ID field on the Customer object
because the Custom Fields array is not populated on a GetList API call (returns null and a seperate API call is required to get Custom Fields) it would be very helpful to have a field on the Customer object for an External ID.
this would mitigate the need for a Custom Field to hold this data and the API would return this value when getting a List of Customers.
This is definitely a need. The other problem with using a Custom Field to hold the external ID is that if you enable the customer portal, the customer can log in and change it--that's very bad!
I don't see how, without an external ID, an external system can properly verify subscription status.