I'm not a ruby dev, but could you be more specific? Assuming you are refering to syntax, not speed: how is it good for prototyping but not "data-intensive tasks"
Beyond that, a lot of the metaprogramming features that make Ruby cool are also expensive when done repetitively.
Also, Ruby's threads REALLY suck. And it has impossible to track down memory leaks in the interpreter. Ask mojombo about fun with those.
Prototyping tasks require efficient programing (as in, the act of putting the program together) above all else.
Ruby excels at the latter, but not (yet) at the former.
For my serious apps, I typically write the front end in JRuby on Rails, and migrate code back into Java as it stabilizes and/or needs more performance. Works great for me!