RPC/encoded format caught on very quickly because it didn't rely on XML Schemas, which were not standard at the time that SOAP 1.0 was released. Now schemas are standard and most vendors support them. Document/literal messages rely on an XML Schema that defines the message format so you can design elaborate messages that use the full power of XML Schema. Such elaborate schemas are usually required when you are exchanging business data across applications or businesses.
The good news is that by default, ASP.NET Web services use document/literal messages unless you explicitly configure them to use one of the other two supported formats (RPC/encoded or RPC/literal). See Chapters 3 and 4