Another great thing about REST is how many free, mature, open source development tools are available to build REST APIs, not to mention the ever evolving power of Web browsers (e.g. adoption of HTML5). It is cheaper for a developer to build a REST API than it often is with SOAP or other, older integration methods (e.g. CORBA).
REST is also easy to transform and proxy.
For example, a vendor may already has a REST API that does everything (and more) that WADO-RS, QIDO, and STOW will define, but the syntax is not exactly the same. So, where a WADO-RS (still being finalized) URL might look like https://server_name/study/x (where x is the StudyUID), the vendor API might expect https://server_name/?StudyUID=x. To develop a WADO-RS compliant API, one can simply develop a very simple method to transpose the received WADO-RS URL to the expected internal URL syntax. The processing time for this type of text transposition is so minimal, it would not be detectable. What does this mean: with a robust REST API, it is simple to adapt to new standards or proprietary methods. And the transformation can be done on either the requesting side (change how you ask), or on the receiving side (change how you listen and answer). It matters more what your API can do, than how to form your URLs.