Relevant URL:
There are two SOAP message styles:
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RPC
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Document Literal
The Document literal form means that the SOAP request (data on the wire) looks and feels like an XML document. Whilst the RPC literal form is an XML representation of a method call (and sending parameters).
The wisdom is that document literal forms are easier to read (from a programming/parser POV) than an RPC call, so it seems to be the preferred style. Its easier to read programmatically so does not require lots of libraries to implement.