"[W]hile XML-RPC and SOAP are inherently symmetric, there is often more value in getting XML out of a service than in sending XML into a service. What’s more, the URL-line remains, for lots of reasons, a really useful way to request services that may or may not emit XML, and if they do emit XML, may or may not emit SOAP or XML-RPC packets."
01/03/02 update: another advantage of url-based web services is that you can call them very easily from browser shortcuts with bookmarklets, eg. to get information from dictionary.com, Blogdex, Daypop, Wayback Machine or Google, or send data to Spyonit and Blogger.
02/18/02 update: [xml-dev] Traditional RPC vs REST.
02/25/02 update: REST and the Real World.