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.

I feel like the list of things "every programmer should know" tends to include "all the things I know,"in which case you may be right.