http://www.fawcette.com/vsm/2002_04/online/online_eprods/webservices_bwagner04_16/default_pf.asp (via Tima)
There are two SOAP message styles:
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.