Maya standalone is a bit more than a render interface.
mayapy.exe is a full blown python runtime. You can use it just as you would any python install.
maya.standalone is a module you can import into the python runtime. When it is initialized it starts up a full Maya session, just with out the UI. You can do anything that the GUI version of Maya can do (except draw UIs).
A lot of our tools are setup so that you can use them in the GUI to operate on the current scene. Or you can run them in batch mode and pass the tool a list of files to process. This is great for exporting assets, cleaning up bloated scenes and in my above example processing mocap data.
The mocap processor is pretty cool. It reads the input files or directories and builds a list of jobs and setups up a job queue. It then spawns four (one per core) new Maya instances (mayapy.exe) which request work from the job queue. This setup meant our animators could process the data a lot quicker. I was hoping to take it to the next level with IncrediBuild, but so far Maya isn't playing nice.