This week's book giveaway is in the Mac OS forum. We're giving away four copies of a choice of "Take Control of Upgrading to Yosemite" or "Take Control of Automating Your Mac" and have Joe Kissell on-line! See this thread for details.
You are right that in this particular case a single element could be enough.
However it is a known convention to have a common root to all inner elements. So even though with one element it is not really helpful, it is still used. It also makes the SOAP more readable as the wrapper element name is based on the method name.
Web Services Performance, Interoperability And Testing Blog
subject: Complex Type versus Simple Type in a WSDL definition