[davidw]

> Let's accept your argument, that each language is a complete toolbox.

You can write a web server with Tcl, Ruby, Python, Java, Erlang, C, or whatever.

You can also create GUI's with all those languages.

Any major language is most likely capable (and has some libraries) for doing common things.

> You'd likely grab a different toolbox to fix different classes of problems.

Like jacquesm says below, you would only do so once the cost of learning a new language is less than the perceived gains from using that new language. Since programmers are expensive, and jumping to something new and potentially unknown has some potential pitfalls, this is often not the case. I.e. it's better for the Ruby guy to write a little web server in Ruby rather than Erlang if that's what he needs, even though the Erlang one will kick the stuffing out of it in performance tests, unless of course he knows that performance is absolutely critical (it isn't in many cases), and more important than solving the "business logic" of the problem.