Honestly I think unit tests and interactive debuggers are really for different purposes: unit tests are more used to guard existing code that has been written,

Honestly I think unit tests and interactive debuggers are really for different purposes: unit tests are more used to guard existing code that has been written, while a debugger can be more useful tinkering with new libraries before writing anything production ready. This is especially true for Ruby(and maybe for Clojure?), since in many cases documentations for libraries are not so great, so you will really have to try the library a little bit to get to the results you want.