The plugin for RenderMan Studio, RenderMan for Maya Pro, has its own internal version of prman. However, the value of having an external version of prman ie. RenderMan Pro-Server, is that you will be able to render rib files that you generate from Maya. Not rendering internally with the plugin (ie. immediate mode) but using deferred rendering with rib files is very advantageous. But of course there is the extra expense of purchasing RenderMan Pro-Server. Either way, you have access to the renderer-of-choice of the feature film industry.
All the best,
Malcolm Kesson