Anything you learn can only benefit you, never is it a bad idea to learn something. That said, if you feel you're productive with Python then I don't see why you would need bash. Python is more powerful (being a full blown programming language) and IMO has a nicer, more readable syntax. Bash is often useful for quick and dirty scripting, though.
If you do decide to learn it, here are some resources to get you started:
- A Bash guide for beginners
- The magnificent Bash FAQ
- Common Bash Pitfalls
Although some rules-of-thumb are occasionally thrown in the air regarding when you should use each, I think you'll figure it out once you get to know all of your options. Having another tool in your arsenal is always an advantage.