The primary appeal of a REST interface is simplicity. You don't need a proprietary library to make sense of it. All you need are a list of resources and in the case of PUT/POST operations, parameters, and the rest is handled by the nature of HTTP. No fancy libraries, no confounded WSDL files, easily-inspected responses. You can consume it using very common libraries and even roll your own with ease.

If you're always providing API access through a proprietary library, it doesn't matter if it's REST, SOAP, custom binary protocols, or whatnot.