Every programmer should learn C during their programming career. Its benefits are to numerous to ignore. Not only will it open many more job opportunities, but it will teach you more about computers as a whole.
1) C is lower level then other programming languages (C++, Java). Programming at a lower level allows you to further your understanding of computers, as a whole.
2) Device drivers and operating systems are written exclusively in C. Now, you may never write a device driver or an operating system, but what if you are ever required to modify one?
3) What if you ever want to get a job programming microcontrollers? They are programmed in C. Are you going to limit your possible list of jobs because you didn't want to learn a new language?
4) C programs are smaller and faster then any other program created in a different language. Sometimes your program needs that speed boost that only C can give it.
5) If you have learned C, you can learn any modern programming language. The reason behind this is that all modern programming languages are based on C (Java, C++, C#, etc).
6) Because C has been around for many years, it has a large community and collective code base. This allows you to quickly and efficiently implement new algorithms or functions that have been programmed before.
7) C is the language of the Open Source community. The Open Source poster child, Linux, was coded in C. If you know C, you can participate in and contribute to numerous Open Source communities like Source Forge.
9) C is still the most commonly required language for programming jobs. It is well worth your time to get C under your belt.
10) Anything that has a microprocessor in it has support for C. From your microwave to your cell phone, C powers technology