I wonder why would a C++, C#, Java developer want to learn a dynamic language?
Assuming the company won't switch its main development language from C++/C#/Java to a dynamic one what use is there for a dynamic language?
What helper tasks can be done by the dynamic languages faster or better after only a few days of learning than with the static language that you have been using for several years?
Update
After seeing the first few responses it is clear that there are two issues.
My main interest would be something that is justifiable to the employer as an expense.
That is, I am looking for justifications for the employer to finance the learning of a dynamic language. Aside from the obvious that the employee will have broader view, the
employers are usually looking for some "real" benefit.
I have often found that learning another language, especially a dynamically typed language, can teach you things about other languages and make you an overall better programmer.  Learning ruby, for example, will teach you Object Oriented programming in ways Java wont, and vice versa.  All in all, I believe that it is better to be a well rounded programmer than stuck in a single language.  It makes you more valuable to the companies/clients you work for.
Why learn Perl, Python, Ruby if the company is using C++, C# or Java as the application language?