A range of addresses can be declared as available without further definition. This allows third parties to use these spare fields for their custom requirements.
If you hope to present your services to other companies this may be a drawback. REST (and "pseudo-rest" variants) has become the de-facto choice, with SOAP a close second. This means anyone connecting to your services would need to familiarise themselves with a new method of communication, and that they are less likely to have an existing toolset for such integrations built into their systems.
The protocol includes versioning, allowing clients on older versions of code to continue to communicate with an upgraded server (i.e. clients and servers don't need to be upgraded in unison).
The data structure's schema is encoded in the message alongside its data, allowing you to easily amend the data structure and have this continue to be understood by other systems. This also avoids the need for versioning, as the data's related structure can be seen directly.