The plumber is effectively at the OS level. It's a userspace program, but it provides it services to the whole system at once. It's a global service that handles all plumbing between all programs. (Think "Android Intents". It's not too far off, conceptually.). Running, for example,
plumb path/to/file.c:123
will send the file to whatever editor instance is set up to handle C files, and move the dot to line 123, from wherever it is invoked, be it the command line, the
plumb bar.jpg
will open bar.jpg in whatever image viewer is appropriate. More or less, you can think of the plumber as a system level "take this text and do an appropriate action with it" service, where you can set up whatever rules you want to do the plumbing.
I don't think there's a good reason for this to live inside the OS, instead of simply running as a service. I don't think there's any extra functionality or elegance that would buy you.
