Why I would pick C over PBasic for anything:

1. From a technical standpoint, C can do far more than PBasic could ever do. You simply can't have as much control over the robot with PBasic as you can with C. PBasic's structure is also very akwardly construed when compared to the majority of languages. We wrote algorithms on our robot this year that we could never have dreamed of putting on last year's PBasic bot, only because we couldn't write them in PBasic.
2. C is the "real programmers'" language (including C++, also). I highly doubt that many professional programmers use PBasic to create real world projects. Remember, FIRST is all about us pursuing careers in science/tech/engineering/etc. so why wouldn't they pick a language that we can use in our careers?
3. From a practical standpoint: Learning PBasic and then trying to use your knowledge as a stepping stone to other languages is very hard. If you know C, however, it is simple to learn C++ or Java, which are far more widely used than PBasic.