Need? No. For accessing resources over HTTP, you can use any other HTTP API, like the HttpURLConnection class, or the HttpClient library. I would imagine that the Restlet client API has some nice utility methods that make it easier, though.
Restlet, Jersey, and JSR-311 (JAX-RS) are server-side frameworks for building RESTful web services. A (test) client emerged with Jersey - though more or less accidentally, that is not in response to JSR-311 (see com.sun.jersey.client.apache for an HttpClient backed version). Personally, I think that HttpClient is probably the best foundation for a client of a RESTful web service - especially as there don't seem to be any tools that consume WADLs to produce client-side proxies.
That being said Restlet does offer a org.restlet.Client class. This is mainly motivated by the desire to hide the standard java client/server class asymmetry (e.g. client-side HttpUrlConnection vs. server-side HttpServletRequest/HttpServletResponse) and replace it with a symmetry where both client and server use org.restlet.data.Request and org.restlet.data.Request.