A one-language programmer is constrained in her thinking by that language.
Moving among these paradigms creates the greatest challenges. Why are these challenges good? That has to do with the way we think about the implementation of algorithms and the idioms and patterns of implementation that apply.
Programmers should always be interested in learning new languages, preferably from an unfamiliar paradigm.
Although it's a start, a one-week training course is not sufficient to learn a new language: it generally takes a good few months of use, even if part-time, to gain a proper working knowledge of a language. It is the idioms of use, not just the syntax and computational model, that are the important factors.