he makes the point RPC is usually used to expose existing components as web services
This is because existing components might have been designed without keeping in mind of potential future exposure as web services, they might well contain "overloaded" methods. As typical RPC style SOAP message relies on dispatching SOAP message based on "method" name, it'd would confuse Web Service API to which method it should dispatch incoming input request.
On the otherhand, Document style SOAP message replies on "order and type" of the arguments to dispatch incoming input request. Unless you are sure existing code doesn't have overloaded methods, you shouldn't use this style to expose as Web Services. Note that "operation overloading" is already NOT allowed by WS Basic Profile 1.0.
(OCEEJBD6, SCWCD5, SCDJWS, SCJP1.4 and Oracle SQL 1Z0-051)