I'm just confused is ACR in Photoshop CS3/4 to be used for RAW images or JPEG's. It almost seems like ACR is just a new layout specific to people editing photo's whereas the main Photoshop layout is non specific and essentially cluttered with art work tools.
Adobe has provided ACR, Lightroom, Bridge and Photoshop to allow photographers different workflows. Someone who shoots a wedding and captures thousands of images will have (and need) a different workflow than a still-life shooter who is capturing dozens of images in order to create a single, final image. Think of these different programs as tools that each have their own strengths and weaknesses in terms of workflow.
In the book, Real World Camera Raw (written by the chap with the heavy beard), he states that Adobe ported several features from Lightroom to ACR in order to help photographers' workflow. This allows several different methods and/or approaches to process images.
Throughout all of Adobe's manuals and tutorials, they state that they want several different methods to solving the same problem because everyone develops their own workflow. Adobe strives to make their software a flexible toolset. Just because something is available doesn't mean it must be used in that way.