When a customer is flagged with 'Ship To Customer' = Y, then I would like access to the Lead Source field.
 
Lead Source is a great field to classify customers with because you choose classifications from a database that provides a meaningfull description.  
 
We use the Class field for a simple A, B, C type classification.  We use the Lead Source field for lead source information until they become an active customer,  then we further classify the type of customer.
 
I understand why the sales type fields are skipped when Ship To is 'Y'.  I also acknowledge that 'Lead Source' is a Sales type column, but, it would not hurt to allow 'Lead Source' to be used on 'Ship To' type customers.