Ask HN: Is there any advantage to using SOAP these days?

Then SOAP is a proper RPC protocol, with clearly specified way of reporting call errors and well-defined serialization method. This is a large advantage, as you don't need to design a protocol every single time you expose an API to the world.

On the other hand, SOAP is a very complicated thing, especially if you count in its all iterations, of which every one changes the paradigm of using SOAP. It's better to stick to its roots, XML-RPC, or more modern variant JSON-RPC (2.0) (or RPC protocols in general). Simple to wrap one's head around, simple to use, and all the necessary things are firmly defined, so programmer's job is only to design the procedures and their arguments.