The arguments about the simplicity of solutions using XML-RPC or REST are easy to understand and hard to argue with.
I have often also heard arguments that the increased overhead of SOAP may significantly impact used bandwidth and possibly even latency. I would like to see the results of a test that quantifies the impact. Any one know a good source for such information?
There are a few studies which have been done regarding this which you might find informative. Please see the following:
There is also an (somewhat out of date) interesting performance conversation about the topic at the MSDN Forums.
In short - most of these sources seem to agree that SOAP and REST are roughly the same performance for general-purpose data. Some results, however, seem to indicate that with binary data, REST may actually be less performant. See the links in the forum I linked for more detail on this.