The shell is available everywhere. If you stick to a relatively basic set of portable functionality, your scripts can run on cell phones, wireless routers, DVRs, netbooks, workstations, big iron servers, and the like. Python is not necessarily included out of the box on lots of systems, and depending on the environment it may be hard to get it installed.
Learning some shell scripting can also help you learn some command line tricks, since the command line is, well, the shell. It's also good for taking some fairly long and complicated command line, and converting that into a more general script after you realize you'll need it some more.
The shell also has some pretty powerful features; pipelines are a really interesting control construct that is native only to the shell, as far as I know.