From my experience working with people, one can be an excellent programmer knowing just one language, whether it is C#, Ruby, or Java. On the other hand, I have met (too many) people who "knew" a lot but who were quite bad at putting that knowledge into practice.
I'm not sure I agree. Maybe the list of things presented by bloggers as things every programmer should know, but C would not be a new addition to such a list in any case.